diff --git a/middleman-core/lib/middleman-core/application.rb b/middleman-core/lib/middleman-core/application.rb
index be560400..38ba9ed2 100644
--- a/middleman-core/lib/middleman-core/application.rb
+++ b/middleman-core/lib/middleman-core/application.rb
@@ -12,10 +12,14 @@ require "middleman-core/vendor/hooks-0.2.0/lib/hooks"
 require "middleman-core/sitemap"
 
 require "middleman-core/core_extensions"
+require "middleman-core/configuration"
 
 # Core Middleman Class
 module Middleman
   class Application
+    # Global configuration
+    include Configuration::Global
+
     # Uses callbacks
     include Hooks
 
@@ -25,115 +29,75 @@ module Middleman
     # Ready (all loading and parsing of extensions complete) hook
     define_hook :ready
 
-    class << self
-
-      # Mix-in helper methods. Accepts either a list of Modules
-      # and/or a block to be evaluated
-      # @return [void]
-      def helpers(*extensions, &block)
-        class_eval(&block)   if block_given?
-        include(*extensions) if extensions.any?
-      end
-
-      # Access class-wide defaults
-      #
-      # @private
-      # @return [Hash] Hash of default values
-      def defaults
-        @defaults ||= {}
-      end
-
-      # Set class-wide defaults
-      #
-      # @param [Symbol] key Unique key name
-      # @param value Default value
-      # @return [void]
-      def set(key, value=nil, &block)
-        @defaults ||= {}
-        @defaults[key] = value
-
-        @inst.set(key, value, &block) if @inst
-      end
-    end
-
-    delegate :helpers, :to => :"self.class"
-
-    # Set attributes (global variables)
-    #
-    # @param [Symbol] key Name of the attribue
-    # @param value Attribute value
+    # Mix-in helper methods. Accepts either a list of Modules
+    # and/or a block to be evaluated
     # @return [void]
-    def set(key, value=nil, &block)
-      setter = "#{key}=".to_sym
-      self.class.send(:attr_accessor, key) if !respond_to?(setter)
-      value = block if block_given?
-      send(setter, value)
+    def self.helpers(*extensions, &block)
+      class_eval(&block)   if block_given?
+      include(*extensions) if extensions.any?
     end
+    delegate :helpers, :to => :"self.class"
 
     # Root project directory (overwritten in middleman build/server)
     # @return [String]
-    set :root,        ENV["MM_ROOT"] || Dir.pwd
+    def self.root
+      ENV["MM_ROOT"] || Dir.pwd
+    end
+    delegate :root, :to => :"self.class"
 
     # Pathname-addressed root
-    def root_path
-      @_root_path ||= Pathname.new(root)
+    def self.root_path
+      Pathname(root)
     end
+    delegate :root_path, :to => :"self.class"
 
     # Name of the source directory
     # @return [String]
-    set :source,      "source"
+    config.define_setting :source,      "source", 'Name of the source directory'
 
     # Middleman environment. Defaults to :development, set to :build by the build process
     # @return [String]
-    set :environment, (ENV['MM_ENV'] && ENV['MM_ENV'].to_sym) || :development
+    config.define_setting :environment, ((ENV['MM_ENV'] && ENV['MM_ENV'].to_sym) || :development), 'Middleman environment. Defaults to :development, set to :build by the build process'
 
     # Which file should be used for directory indexes
     # @return [String]
-    set :index_file,  "index.html"
+    config.define_setting :index_file,  "index.html", 'Which file should be used for directory indexes'
 
     # Whether to strip the index file name off links to directory indexes
     # @return [Boolean]
-    set :strip_index_file, true
+    config.define_setting :strip_index_file, true, 'Whether to strip the index file name off links to directory indexes'
 
     # Whether to include a trailing slash when stripping the index file
     # @return [Boolean]
-    set :trailing_slash, true
+    config.define_setting :trailing_slash, true, 'Whether to include a trailing slash when stripping the index file'
 
     # Location of javascripts within source.
     # @return [String]
-    set :js_dir,      "javascripts"
+    config.define_setting :js_dir,      "javascripts", 'Location of javascripts within source'
 
     # Location of stylesheets within source. Used by Compass.
     # @return [String]
-    set :css_dir,     "stylesheets"
+    config.define_setting :css_dir,     "stylesheets", 'Location of stylesheets within source'
 
     # Location of images within source. Used by HTML helpers and Compass.
     # @return [String]
-    set :images_dir,  "images"
+    config.define_setting :images_dir,  "images", 'Location of images within source'
 
     # Location of fonts within source. Used by Compass.
     # @return [String]
-    set :fonts_dir,   "fonts"
+    config.define_setting :fonts_dir,   "fonts", 'Location of fonts within source'
 
     # Where to build output files
     # @return [String]
-    set :build_dir,   "build"
+    config.define_setting :build_dir,   "build", 'Where to build output files'
 
     # Default prefix for building paths. Used by HTML helpers and Compass.
     # @return [String]
-    set :http_prefix, "/"
-
-    # Default string encoding for templates and output.
-    # @return [String]
-    set :encoding,    "utf-8"
-
-    # Whether to catch and display exceptions
-    # @return [Boolean]
-    set :show_exceptions, true
+    config.define_setting :http_prefix, "/", 'Default prefix for building paths'
 
     # Default layout name
     # @return [String, Symbold]
-    set :layout, :_auto_layout
+    config.define_setting :layout, :_auto_layout, 'Default layout name'
 
     # Activate custom features and extensions
     include Middleman::CoreExtensions::Extensions
@@ -178,12 +142,12 @@ module Middleman
       cache.clear
 
       # Setup the default values from calls to set before initialization
-      self.class.superclass.defaults.each { |k,v| set(k,v) }
+      self.class.superclass.config.to_h.each { |k,v| self.class.config.define_setting(k,v) }
 
       # Evaluate a passed block if given
       instance_exec(&block) if block_given?
 
-      set :source, ENV["MM_SOURCE"] if ENV["MM_SOURCE"]
+      config[:source] = ENV["MM_SOURCE"] if ENV["MM_SOURCE"]
 
       super
     end
@@ -199,24 +163,17 @@ module Middleman
 
     # Whether we're in development mode
     # @return [Boolean] If we're in dev mode
-    def development?; environment == :development; end
+    def development?; config[:environment] == :development; end
 
     # Whether we're in build mode
     # @return [Boolean] If we're in build mode
-    def build?; environment == :build; end
-
-    # Backwards compatibilty with old Sinatra template interface
-    #
-    # @return [Middleman::Application]
-    def settings
-      self
-    end
+    def build?; config[:environment] == :build; end
 
     # The full path to the source directory
     #
     # @return [String]
     def source_dir
-      File.join(root, source)
+      File.join(root, config[:source])
     end
 
     delegate :logger, :instrument, :to => ::Middleman::Util
@@ -239,7 +196,7 @@ module Middleman
 
       if !resource
         # Try it with /index.html at the end
-        indexed_path = File.join(path.sub(%r{/$}, ''), index_file)
+        indexed_path = File.join(path.sub(%r{/$}, ''), config[:index_file])
         resource = sitemap.find_resource_by_destination_path(indexed_path)
       end
 
diff --git a/middleman-core/lib/middleman-core/cli/build.rb b/middleman-core/lib/middleman-core/cli/build.rb
index 7ec7a558..6b9dcb5a 100644
--- a/middleman-core/lib/middleman-core/cli/build.rb
+++ b/middleman-core/lib/middleman-core/cli/build.rb
@@ -83,7 +83,7 @@ module Middleman::Cli
       # @return [Middleman::Application]
       def shared_instance(verbose=false, instrument=false)
         @_shared_instance ||= ::Middleman::Application.server.inst do
-          set :environment, :build
+          config[:environment] = :build
           logger(verbose ? 0 : 1, instrument)
         end
       end
@@ -114,7 +114,7 @@ module Middleman::Cli
       # @param [Middleman::Sitemap::Resource] resource
       # @return [String] The full path of the file that was written
       def render_to_file(resource)
-        build_dir = self.class.shared_instance.build_dir
+        build_dir = self.class.shared_instance.config[:build_dir]
         output_file = File.join(build_dir, resource.destination_path)
 
         begin
diff --git a/middleman-core/lib/middleman-core/configuration.rb b/middleman-core/lib/middleman-core/configuration.rb
new file mode 100644
index 00000000..6cede94d
--- /dev/null
+++ b/middleman-core/lib/middleman-core/configuration.rb
@@ -0,0 +1,231 @@
+module Middleman
+  module Configuration
+    # Access to a global configuration manager for the whole Middleman project, 
+    # plus backwards compatibility mechanisms for older Middleman projects.
+    module Global
+      def self.included(app)
+        app.send :extend, ClassMethods
+      end
+
+      module ClassMethods
+        # Global configuration for the whole Middleman project.
+        # @return [ConfigurationManager]
+        def config
+          @_config ||= ConfigurationManager.new
+        end
+
+        # Set attributes (global variables)
+        #
+        # @deprecated Prefer accessing settings through "config".
+        #
+        # @param [Symbol] key Name of the attribue
+        # @param value Attribute value
+        # @return [void]
+        def set(key, default=nil, &block)
+          config.define_setting(key, default)
+          @inst.set(key, default, &block) if @inst
+        end
+
+        # Access global settings as methods, to preserve compatibility with
+        # old Middleman.
+        #
+        # @deprecated Prefer accessing settings through "config".
+        def method_missing(method, *args)
+          if config.defines_setting? method
+            config[method]
+          else
+            super
+          end
+        end
+
+        # Needed so that method_missing makes sense
+        def respond_to?(method, include_private = false)
+          super || config.defines_setting?(method)
+        end
+      end
+
+      def config
+        self.class.config
+      end
+
+      # Backwards compatibilty with old Sinatra template interface
+      #
+      # @deprecated Prefer accessing settings through "config".
+      #
+      # @return [ConfigurationManager]
+      alias :settings :config
+
+      # Set attributes (global variables)
+      #
+      # @deprecated Prefer accessing settings through "config".
+      #
+      # @param [Symbol] key Name of the attribue
+      # @param value Attribute value
+      # @return [void]
+      def set(key, value=nil, &block)
+        value = block if block_given?
+        config[key] = value
+      end
+
+      # Access global settings as methods, to preserve compatibility with
+      # old Middleman.
+      #
+      # @deprecated Prefer accessing settings through "config".
+      def method_missing(method, *args)
+        if config.defines_setting? method
+          config[method]
+        else
+          super
+        end
+      end
+
+      # Needed so that method_missing makes sense
+      def respond_to?(method, include_private = false)
+        super || config.defines_setting?(method)
+      end
+    end
+
+    # A class that manages a collection of documented settings.
+    # Can be used by extensions as well as the main Middleman
+    # application. Extensions should probably finalize their instance
+    # after defining all the settings they want to expose.
+    class ConfigurationManager
+      def initialize
+        # A hash from setting key to ConfigSetting instance.
+        @settings = {}
+        @finalized = false
+      end
+
+      # Get all settings, sorted by key, as ConfigSetting objects.
+      # @return [Array<ConfigSetting>]
+      def all_settings
+        @settings.values.sort_by(&:key)
+      end
+
+      # Get a full ConfigSetting object for the setting with the give key.
+      # @return [ConfigSetting]
+      def setting(key)
+        @settings[key]
+      end
+
+      # Get the value of a setting by key. Returns nil if there is no such setting.
+      # @return [Object]
+      def [](key)
+        setting = @settings[key]
+        setting ? setting.value : nil
+      end
+
+      # Set the value of a setting by key. Creates the setting if it doesn't exist.
+      # @param [Symbol] key
+      # @param [Object] val
+      def []=(key, val)
+        setting = @settings[key] || define_setting(key)
+        setting.value = val
+      end
+
+      # Allow configuration settings to be read and written via methods
+      def method_missing(method, *args)
+        if defines_setting?(method) && args.size == 0
+          self[method]
+        elsif method =~ /^(\w+)=$/ && args.size == 1
+          self[$1.to_sym] = args[0]
+        else
+          super
+        end
+      end
+
+      # Needed so that method_missing makes sense
+      def respond_to?(method, include_private = false)
+        super || defines_setting?(method) || (method =~ /^(\w+)=$/ && defines_setting?($1))
+      end
+
+      # Does this configuration manager know about the setting identified by key?
+      # @param [Symbol] key
+      # @return [Boolean]
+      def defines_setting?(key)
+        @settings.has_key?(key)
+      end
+
+      # Define a new setting, with optional default and user-friendly description.
+      # Once the configuration manager is finalized, no new settings may be defined.
+      #
+      # @param [Symbol] key
+      # @param [Object] default
+      # @param [String] description
+      # @return [ConfigSetting]
+      def define_setting(key, default=nil, description=nil)
+        raise "Setting #{key} doesn't exist" if @finalized
+        raise "Setting #{key} already defined" if @settings.has_key?(key)
+        raise "Setting key must be a Symbol" unless key.is_a? Symbol
+
+        @settings[key] = ConfigSetting.new(key, default, description)
+      end
+
+      # Switch the configuration manager is finalized, it switches to read-only
+      # mode and no new settings may be defined.
+      def finalize!
+        @finalized = true
+      end
+
+      # Deep duplicate of the configuration manager
+      def dup
+        copy = ConfigurationManager.new
+        @settings.each do |key, setting|
+          copy_setting = copy.define_setting setting.key, setting.default, setting.description
+          copy_setting.value = setting.value if setting.value_set?
+        end
+        copy
+      end
+
+      def to_h
+        hash = {}
+        @settings.each do |key, setting|
+          hash[key] = setting.value
+        end
+        hash
+      end
+
+      def to_s
+        to_h.inspect
+      end
+    end
+
+    # An individual configuration setting, with an optional default and description.
+    # Also models whether or not a value has been set.
+    class ConfigSetting
+      # The name of this setting
+      attr_accessor :key
+
+      # The default value for this setting
+      attr_accessor :default
+
+      # A human-friendly description of the setting
+      attr_accessor :description
+
+      def initialize(key, default, description)
+        @value_set = false
+        self.key = key
+        self.default = default
+        self.description = description
+      end
+
+      # The user-supplied value for this setting, overriding the default
+      def value=(value)
+        @value = value
+        @value_set = true
+      end
+
+      # The effective value of the setting, which may be the default
+      # if the user has not set a value themselves. Note that even if the
+      # user sets the value to nil it will override the default.
+      def value
+        value_set? ? @value : default
+      end
+
+      # Whether or not there has been a value set beyond the default
+      def value_set?
+        @value_set
+      end
+    end
+  end
+end
diff --git a/middleman-core/lib/middleman-core/core_extensions/data.rb b/middleman-core/lib/middleman-core/core_extensions/data.rb
index 360d953f..bc2778af 100644
--- a/middleman-core/lib/middleman-core/core_extensions/data.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/data.rb
@@ -13,7 +13,7 @@ module Middleman
           require "yaml"
           require "active_support/json"
 
-          app.set :data_dir, "data"
+          app.config.define_setting :data_dir, "data", "The directory data files are stored in"
           app.send :include, InstanceMethods
         end
         alias :included :registered
@@ -24,12 +24,12 @@ module Middleman
         # Setup data files before anything else so they are available when
         # parsing config.rb
         def initialize
-          self.files.changed DataStore.matcher do |file|
-            self.data.touch_file(file) if file.match(%r{^#{self.data_dir}\/})
+          files.changed DataStore.matcher do |file|
+            data.touch_file(file) if file.match(%r{^#{config[:data_dir]}\/})
           end
 
-          self.files.deleted DataStore.matcher do |file|
-            self.data.remove_file(file) if file.match(%r{^#{self.data_dir}\/})
+          files.deleted DataStore.matcher do |file|
+            data.remove_file(file) if file.match(%r{^#{config[data_dir]}\/})
           end
 
           super
diff --git a/middleman-core/lib/middleman-core/core_extensions/extensions.rb b/middleman-core/lib/middleman-core/core_extensions/extensions.rb
index e10359ab..3f8d1d01 100644
--- a/middleman-core/lib/middleman-core/core_extensions/extensions.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/extensions.rb
@@ -37,19 +37,13 @@ module Middleman
       class << self
         # @private
         def registered(app)
-          # Using for version parsing
-          require "rubygems"
-
           app.define_hook :after_configuration
           app.define_hook :before_configuration
           app.define_hook :build_config
           app.define_hook :development_config
 
-          if ENV["AUTOLOAD_SPROCKETS"]
-            app.set :autoload_sprockets, (ENV["AUTOLOAD_SPROCKETS"] == "true")
-          else
-            app.set :autoload_sprockets, true
-          end
+          app.config.define_setting :autoload_sprockets, true, 'Automatically load sprockets at startup?'
+          app.config[:autoload_sprockets] = (ENV["AUTOLOAD_SPROCKETS"] == "true") if ENV["AUTOLOAD_SPROCKETS"]
 
           app.extend ClassMethods
           app.send :include, InstanceMethods
@@ -137,7 +131,7 @@ module Middleman
             instance_eval File.read(local_config), local_config, 1
           end
 
-          if autoload_sprockets
+          if config[:autoload_sprockets]
             begin
               require "middleman-sprockets"
               activate(:sprockets)
diff --git a/middleman-core/lib/middleman-core/core_extensions/external_helpers.rb b/middleman-core/lib/middleman-core/core_extensions/external_helpers.rb
index 0f2118ea..6907740b 100644
--- a/middleman-core/lib/middleman-core/core_extensions/external_helpers.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/external_helpers.rb
@@ -9,20 +9,20 @@ module Middleman
         # once registered
         def registered(app)
           # Setup a default helpers paths
-          app.set :helpers_dir, "helpers"
-          app.set :helpers_filename_glob, "**{,/*/**}/*.rb"
-          app.set :helpers_filename_to_module_name_proc, Proc.new { |filename|
+          app.config.define_setting :helpers_dir, "helpers", 'Directory to autoload helper modules from'
+          app.config.define_setting :helpers_filename_glob, "**.rb", 'Glob pattern for matching helper ruby files'
+          app.config.define_setting :helpers_filename_to_module_name_proc, Proc.new { |filename|
             basename = File.basename(filename, File.extname(filename))
             basename.camelcase
-          }
+          }, 'Proc implementing the conversion from helper filename to module name'
 
           # After config
           app.after_configuration do
-            helpers_path = File.expand_path(helpers_dir, root)
+            helpers_path = File.join(root, config[:helpers_dir])
             next unless File.exists?(helpers_path)
 
-            Dir[File.join(helpers_path, helpers_filename_glob)].each do |filename|
-              module_name = helpers_filename_to_module_name_proc.call(filename)
+            Dir[File.join(helpers_path, config[:helpers_filename_glob])].each do |filename|
+              module_name = config[:helpers_filename_to_module_name_proc].call(filename)
               next unless module_name
 
               require filename
diff --git a/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb b/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
index 1883b57b..a55471d9 100644
--- a/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
@@ -29,7 +29,7 @@ module Middleman
 
           # Before parsing config, load the data/ directory
           app.before_configuration do
-            files.reload_path(data_dir)
+            files.reload_path(config[:data_dir])
           end
 
           # After config, load everything else
diff --git a/middleman-core/lib/middleman-core/core_extensions/front_matter.rb b/middleman-core/lib/middleman-core/core_extensions/front_matter.rb
index bfc6f87c..f021dde8 100644
--- a/middleman-core/lib/middleman-core/core_extensions/front_matter.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/front_matter.rb
@@ -62,7 +62,7 @@ module Middleman::CoreExtensions
       def clear_data(file)
         # Copied from Sitemap::Store#file_to_path, but without
         # removing the file extension
-        file = File.expand_path(file, @app.root)
+        file = File.join(@app.root, file)
         prefix = @app.source_dir.sub(/\/$/, "") + "/"
         return unless file.include?(prefix)
         path = file.sub(prefix, "")
diff --git a/middleman-core/lib/middleman-core/core_extensions/rendering.rb b/middleman-core/lib/middleman-core/core_extensions/rendering.rb
index cfca198f..2836a105 100644
--- a/middleman-core/lib/middleman-core/core_extensions/rendering.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/rendering.rb
@@ -257,7 +257,7 @@ module Middleman
             extension_class = ::Tilt[ext]
             ::Tilt.mappings.each do |ext, engines|
               next unless engines.include? extension_class
-              engine_options = respond_to?(ext.to_sym) ? send(ext.to_sym) : {}
+              engine_options = config[ext.to_sym] || {}
               options.merge!(engine_options)
             end
 
@@ -272,7 +272,7 @@ module Middleman
         # @return [String]
         def fetch_layout(engine, opts)
           # The layout name comes from either the system default or the options
-          local_layout = opts.has_key?(:layout) ? opts[:layout] : layout
+          local_layout = opts.has_key?(:layout) ? opts[:layout] : config[:layout]
           return false unless local_layout
 
           # Look for engine-specific options
diff --git a/middleman-core/lib/middleman-core/core_extensions/routing.rb b/middleman-core/lib/middleman-core/core_extensions/routing.rb
index 53f3c3ed..ba5c7e7b 100644
--- a/middleman-core/lib/middleman-core/core_extensions/routing.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/routing.rb
@@ -28,12 +28,12 @@ module Middleman
         # @param [String, Symbol] layout_name
         # @return [void]
         def with_layout(layout_name, &block)
-          old_layout = layout
+          old_layout = config[:layout]
 
-          set :layout, layout_name
+          config[:layout] = layout_name
           instance_exec(&block) if block_given?
         ensure
-          set :layout, old_layout
+          config[:layout] = old_layout
         end
 
         # The page method allows the layout to be set on a specific path
@@ -48,7 +48,7 @@ module Middleman
           blocks = Array(block)
 
           # Default layout
-          opts[:layout] = layout if opts[:layout].nil?
+          opts[:layout] = config[:layout] if opts[:layout].nil?
 
           # If the url is a regexp
           if url.is_a?(Regexp) || url.include?("*")
@@ -64,7 +64,7 @@ module Middleman
           # Normalized path
           url = '/' + Middleman::Util.normalize_path(url)
           if url.end_with?('/') || File.directory?(File.join(source_dir, url))
-            url = File.join(url, index_file)
+            url = File.join(url, config[:index_file])
           end
 
           # Setup proxy
diff --git a/middleman-core/lib/middleman-core/core_extensions/ruby_encoding.rb b/middleman-core/lib/middleman-core/core_extensions/ruby_encoding.rb
index e9168386..3ff70b13 100644
--- a/middleman-core/lib/middleman-core/core_extensions/ruby_encoding.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/ruby_encoding.rb
@@ -6,6 +6,10 @@ module Middleman::CoreExtensions::RubyEncoding
 
     # Once registerd
     def registered(app)
+      # Default string encoding for templates and output.
+      # @return [String]
+      app.config.define_setting :encoding,    "utf-8", 'Default string encoding for templates and output'
+
       app.send :include, InstanceMethods
     end
 
@@ -15,8 +19,8 @@ module Middleman::CoreExtensions::RubyEncoding
   module InstanceMethods
     def initialize
       if Object.const_defined?(:Encoding)
-        Encoding.default_internal = encoding
-        Encoding.default_external = encoding
+        Encoding.default_internal = config[:encoding]
+        Encoding.default_external = config[:encoding]
       end
 
       super
diff --git a/middleman-core/lib/middleman-core/core_extensions/show_exceptions.rb b/middleman-core/lib/middleman-core/core_extensions/show_exceptions.rb
index 9dd27fb8..8c34612a 100644
--- a/middleman-core/lib/middleman-core/core_extensions/show_exceptions.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/show_exceptions.rb
@@ -11,10 +11,14 @@ module Middleman
 
         # Once registered
         def registered(app)
+          # Whether to catch and display exceptions
+          # @return [Boolean]
+          app.config.define_setting :show_exceptions, true, 'Whether to catch and display exceptions'
+
           # When in dev
           app.configure :development do
             # Include middlemare
-            if show_exceptions
+            if config[:show_exceptions]
               use ::Middleman::CoreExtensions::ShowExceptions::Middleware
             end
           end
diff --git a/middleman-core/lib/middleman-core/preview_server.rb b/middleman-core/lib/middleman-core/preview_server.rb
index 2d6f187d..0d6b3bec 100644
--- a/middleman-core/lib/middleman-core/preview_server.rb
+++ b/middleman-core/lib/middleman-core/preview_server.rb
@@ -71,7 +71,7 @@ module Middleman
         opts = @options
         @app =::Middleman::Application.server.inst do
           if opts[:environment]
-            set :environment, opts[:environment].to_sym
+            config[:environment] = opts[:environment].to_sym
           end
           
           logger(opts[:debug] ? 0 : 1, opts[:instrumenting] || false)
diff --git a/middleman-core/lib/middleman-core/renderers/erb.rb b/middleman-core/lib/middleman-core/renderers/erb.rb
index 6ae858bb..e6a6f6b6 100644
--- a/middleman-core/lib/middleman-core/renderers/erb.rb
+++ b/middleman-core/lib/middleman-core/renderers/erb.rb
@@ -8,8 +8,8 @@ module Middleman
         # once registered
         def registered(app)
           # Setup a default ERb engine
-          app.set :erb_engine, :erb
-          app.set :erb_engine_prefix, ::Tilt
+          app.config.define_setting :erb_engine, :erb, 'The engine to use for rendering ERb templates'
+          app.config.define_setting :erb_engine_prefix, ::Tilt, 'The parent module for ERb template engines'
 
           app.before_configuration do
             template_extensions :erb => :html
@@ -19,14 +19,14 @@ module Middleman
           app.after_configuration do
             # Find the user's prefered engine
             # Convert symbols to classes
-            if erb_engine.is_a? Symbol
+            if config[:erb_engine].is_a? Symbol
               engine = engine.to_s
               engine = engine == "erb" ? "ERB" : engine.camelize
-              erb_engine = erb_engine_prefix.const_get("#{engine}Template")
+              config[:erb_engine] = config[:erb_engine_prefix].const_get("#{engine}Template")
             end
 
             # Tell Tilt to use the preferred engine
-            ::Tilt.prefer(erb_engine)
+            ::Tilt.prefer(config[:erb_engine])
           end
         end
         alias :included :registered
diff --git a/middleman-core/lib/middleman-core/renderers/less.rb b/middleman-core/lib/middleman-core/renderers/less.rb
index b4382d6d..6784e54c 100644
--- a/middleman-core/lib/middleman-core/renderers/less.rb
+++ b/middleman-core/lib/middleman-core/renderers/less.rb
@@ -11,15 +11,15 @@ module Middleman
 
         # Once registered
         def registered(app)
-          # Default sass options
-          app.set :less, {}
+          # Default less options
+          app.config.define_setting :less, {}, 'LESS compiler options'
 
           app.before_configuration do
             template_extensions :less => :css
           end
 
           app.after_configuration do
-            ::Less.paths << File.expand_path(css_dir, source_dir)
+            ::Less.paths << File.join(source_dir, config[:css_dir])
           end
 
           # Tell Tilt to use it as well (for inline sass blocks)
diff --git a/middleman-core/lib/middleman-core/renderers/markdown.rb b/middleman-core/lib/middleman-core/renderers/markdown.rb
index ae2dd78c..06d017cb 100644
--- a/middleman-core/lib/middleman-core/renderers/markdown.rb
+++ b/middleman-core/lib/middleman-core/renderers/markdown.rb
@@ -10,8 +10,8 @@ module Middleman
         # Once registered
         def registered(app)
           # Set our preference for a markdown engine
-          app.set :markdown_engine, :maruku
-          app.set :markdown_engine_prefix, ::Tilt
+          app.config.define_setting :markdown_engine, :maruku, 'Preferred markdown engine'
+          app.config.define_setting :markdown_engine_prefix, ::Tilt, 'The parent module for markdown template engines'
 
           app.before_configuration do
             template_extensions :markdown => :html,
@@ -26,24 +26,24 @@ module Middleman
 
             begin
               # Look for the user's preferred engine
-              if markdown_engine == :redcarpet
+              if config[:markdown_engine] == :redcarpet
                 require "middleman-core/renderers/redcarpet"
                 ::Tilt.prefer(::Middleman::Renderers::RedcarpetTemplate)
-              elsif !markdown_engine.nil?
+              elsif !config[:markdown_engine].nil?
                 # Map symbols to classes
-                markdown_engine_klass = if markdown_engine.is_a? Symbol
-                  engine = markdown_engine.to_s
+                markdown_engine_klass = if config[:markdown_engine].is_a? Symbol
+                  engine = config[:markdown_engine].to_s
                   engine = engine == "rdiscount" ? "RDiscount" : engine.camelize
-                  markdown_engine_prefix.const_get("#{engine}Template")
+                  config[:markdown_engine_prefix].const_get("#{engine}Template")
                 else
-                  markdown_engine_prefix
+                  config[:markdown_engine_prefix]
                 end
 
                 # Tell tilt to use that engine
                 ::Tilt.prefer(markdown_engine_klass)
               end
             rescue LoadError
-              logger.warn "Requested Markdown engine (#{markdown_engine}) not found. Maybe the gem needs to be installed and required?"
+              logger.warn "Requested Markdown engine (#{config[:markdown_engine]}) not found. Maybe the gem needs to be installed and required?"
             end
           end
         end
diff --git a/middleman-core/lib/middleman-core/renderers/sass.rb b/middleman-core/lib/middleman-core/renderers/sass.rb
index c77ff611..38b6afa1 100644
--- a/middleman-core/lib/middleman-core/renderers/sass.rb
+++ b/middleman-core/lib/middleman-core/renderers/sass.rb
@@ -12,16 +12,16 @@ module Middleman
         # Once registered
         def registered(app)
           # Default sass options
-          app.set :sass, {}
+          app.config.define_setting :sass, {}, 'Sass engine options'
 
           # Location of SASS .sass_cache directory.
           # @return [String]
-          #   set :sass_cache_path, "/tmp/middleman-app-name/sass_cache"
-          app.set(:sass_cache_path) { File.join(app.root_path, '.sass_cache') } # runtime compile of path
+          app.config.define_setting :sass_cache_path, nil, 'Location of sass cache' # runtime compile of path
 
           app.before_configuration do
             template_extensions :scss => :css,
                                 :sass => :css
+            config[:sass_cache_path] = File.join(app.root_path, '.sass_cache')
           end
 
           # Tell Tilt to use it as well (for inline sass blocks)
@@ -76,11 +76,11 @@ module Middleman
           more_opts = { :filename => eval_file, :line => line, :syntax => syntax }
           
           if @context.is_a?(::Middleman::Application) && file
-            location_of_sass_file = File.expand_path(@context.source, @context.root)
+            location_of_sass_file = @context.source_dir
           
             parts = basename.split('.')
             parts.pop
-            more_opts[:css_filename] = File.join(location_of_sass_file, @context.css_dir, parts.join("."))
+            more_opts[:css_filename] = File.join(location_of_sass_file, @context.config[:css_dir], parts.join("."))
           end
           
           options.merge(more_opts)
diff --git a/middleman-core/lib/middleman-core/sitemap.rb b/middleman-core/lib/middleman-core/sitemap.rb
index 3b7c3ef1..a5693630 100644
--- a/middleman-core/lib/middleman-core/sitemap.rb
+++ b/middleman-core/lib/middleman-core/sitemap.rb
@@ -20,10 +20,10 @@ module Middleman
         app.register Middleman::Sitemap::Extensions::Ignores
 
         # Set to automatically convert some characters into a directory
-        app.set :automatic_directory_matcher, nil
+        app.config.define_setting :automatic_directory_matcher, nil, 'Set to automatically convert some characters into a directory'
 
         # Setup callbacks which can exclude paths from the sitemap
-        app.set :ignored_sitemap_matchers, {
+        app.config.define_setting :ignored_sitemap_matchers, {
           # dotfiles and folders in the root
           :root_dotfiles => proc { |file| file.match(%r{^\.}) },
 
@@ -38,7 +38,7 @@ module Middleman
           :layout => proc { |file|
             file.match(%r{^source/layout\.}) || file.match(%r{^source/layouts/})
           }
-        }
+        }, 'Callbacks that can exclude paths from the sitemap'
 
         # Include instance methods
         app.send :include, InstanceMethods
diff --git a/middleman-core/lib/middleman-core/sitemap/extensions/on_disk.rb b/middleman-core/lib/middleman-core/sitemap/extensions/on_disk.rb
index 52600ed7..d311973d 100644
--- a/middleman-core/lib/middleman-core/sitemap/extensions/on_disk.rb
+++ b/middleman-core/lib/middleman-core/sitemap/extensions/on_disk.rb
@@ -45,7 +45,7 @@ module Middleman
           path = @sitemap.file_to_path(file)
           return false unless path
 
-          ignored = @app.ignored_sitemap_matchers.any? do |name, callback|
+          ignored = @app.config[:ignored_sitemap_matchers].any? do |name, callback|
             callback.call(file)
           end
 
@@ -85,7 +85,7 @@ module Middleman
             ::Middleman::Sitemap::Resource.new(
               @sitemap,
               @sitemap.file_to_path(file),
-              File.expand_path(file, @app.root)
+              File.join(@app.root, file)
             )
           end
         end
diff --git a/middleman-core/lib/middleman-core/sitemap/store.rb b/middleman-core/lib/middleman-core/sitemap/store.rb
index 6eb0d300..5108b9a7 100644
--- a/middleman-core/lib/middleman-core/sitemap/store.rb
+++ b/middleman-core/lib/middleman-core/sitemap/store.rb
@@ -152,7 +152,7 @@ module Middleman
       # @param [String] file
       # @return [String]
       def file_to_path(file)
-        file = File.expand_path(file, @app.root)
+        file = File.join(@app.root, file)
 
         prefix = @app.source_dir.sub(/\/$/, "") + "/"
         return false unless file.start_with?(prefix)
@@ -160,8 +160,8 @@ module Middleman
         path = file.sub(prefix, "")
 
         # Replace a file name containing automatic_directory_matcher with a folder
-        unless @app.automatic_directory_matcher.nil?
-          path = path.gsub(@app.automatic_directory_matcher, "/")
+        unless @app.config[:automatic_directory_matcher].nil?
+          path = path.gsub(@app.config[:automatic_directory_matcher], "/")
         end
 
         extensionless_path(path)
diff --git a/middleman-core/lib/middleman-core/step_definitions/builder_steps.rb b/middleman-core/lib/middleman-core/step_definitions/builder_steps.rb
index be1bb082..e119568d 100644
--- a/middleman-core/lib/middleman-core/step_definitions/builder_steps.rb
+++ b/middleman-core/lib/middleman-core/step_definitions/builder_steps.rb
@@ -10,9 +10,12 @@ end
 Given /^an empty app$/ do
   step %Q{a directory named "empty_app"}
   step %Q{I cd to "empty_app"}
+  ENV["MM_ROOT"] = nil
 end
 
 Given /^a fixture app "([^\"]*)"$/ do |path|
+  ENV["MM_ROOT"] = nil
+
   # This step can be reentered from several places but we don't want
   # to keep re-copying and re-cd-ing into ever-deeper directories
   next if File.basename(current_dir) == path
diff --git a/middleman-core/lib/middleman-core/step_definitions/server_steps.rb b/middleman-core/lib/middleman-core/step_definitions/server_steps.rb
index 8a0997f5..ac0a7c3c 100644
--- a/middleman-core/lib/middleman-core/step_definitions/server_steps.rb
+++ b/middleman-core/lib/middleman-core/step_definitions/server_steps.rb
@@ -40,9 +40,10 @@ Given /^the Server is running$/ do
     ENV["MM_SOURCE"] = ""
   end
 
+  ENV["MM_ROOT"] = root_dir
+
   initialize_commands = @initialize_commands || []
   initialize_commands.unshift lambda {
-    set :root, root_dir
     set :environment, @current_env || :development
     set :show_exceptions, false
   }
diff --git a/middleman-core/lib/middleman-core/templates/extension/lib/lib.rb b/middleman-core/lib/middleman-core/templates/extension/lib/lib.rb
index a2459194..1eb1796c 100644
--- a/middleman-core/lib/middleman-core/templates/extension/lib/lib.rb
+++ b/middleman-core/lib/middleman-core/templates/extension/lib/lib.rb
@@ -7,9 +7,6 @@ module MyExtension
 
     # Called when user `activate`s your extension
     def registered(app, options={})
-      # Setup extension-specific config
-      app.set :config_variable, false
-
       # Include class methods
       # app.extend ClassMethods
 
@@ -18,9 +15,6 @@ module MyExtension
 
       app.after_configuration do
         # Do something
-
-        # config_variable is now either the default or the user's
-        # setting from config.rb
       end
     end
     alias :included :registered
diff --git a/middleman-more/lib/middleman-more/core_extensions/assets.rb b/middleman-more/lib/middleman-more/core_extensions/assets.rb
index 13fb0a6f..8fdde3ca 100644
--- a/middleman-more/lib/middleman-more/core_extensions/assets.rb
+++ b/middleman-more/lib/middleman-more/core_extensions/assets.rb
@@ -33,7 +33,7 @@ module Middleman
             if resource = sitemap.find_resource_by_path(path)
               resource.url
             else
-              File.join(http_prefix, path)
+              File.join(config[:http_prefix], path)
             end
           end
         end
diff --git a/middleman-more/lib/middleman-more/core_extensions/compass.rb b/middleman-more/lib/middleman-more/core_extensions/compass.rb
index 2f9f98dd..78b7d13f 100644
--- a/middleman-more/lib/middleman-more/core_extensions/compass.rb
+++ b/middleman-more/lib/middleman-more/core_extensions/compass.rb
@@ -19,38 +19,38 @@ module Middleman
 
           # Location of SASS/SCSS files external to source directory.
           # @return [Array]
-          #   set :sass_assets_paths, ["#{root}/assets/sass/", "/path/2/external/sass/repository/"]
-          app.set :sass_assets_paths, []
+          #   config[:sass_assets_paths] = ["#{root}/assets/sass/", "/path/2/external/sass/repository/"]
+          app.config.define_setting :sass_assets_paths, [], 'Paths to extra SASS/SCSS files'
 
           app.after_configuration do
-            ::Compass.configuration do |config|
-              config.project_path    = source_dir
-              config.environment     = :development
-              config.cache_path      = sass_cache_path
-              config.sass_dir        = css_dir
-              config.additional_import_paths = sass_assets_paths
-              config.css_dir         = css_dir
-              config.javascripts_dir = js_dir
-              config.fonts_dir       = fonts_dir
-              config.images_dir      = images_dir
-              config.http_path       = http_prefix
+            ::Compass.configuration do |compass_config|
+              compass_config.project_path    = source_dir
+              compass_config.environment     = :development
+              compass_config.cache_path      = config[:sass_cache_path]
+              compass_config.sass_dir        = config[:css_dir]
+              compass_config.additional_import_paths = config[:sass_assets_paths]
+              compass_config.css_dir         = config[:css_dir]
+              compass_config.javascripts_dir = config[:js_dir]
+              compass_config.fonts_dir       = config[:fonts_dir]
+              compass_config.images_dir      = config[:images_dir]
+              compass_config.http_path       = config[:http_prefix]
 
               # Disable this initially, the cache_buster extension will
               # re-enable it if requested.
-              config.asset_cache_buster :none
+              compass_config.asset_cache_buster :none
 
               # Disable this initially, the relative_assets extension will
               # re-enable it if requested.
-              config.relative_assets = false
+              compass_config.relative_assets = false
 
               # Default output style
-              config.output_style = :nested
+              compass_config.output_style = :nested
 
               # No line-comments in test mode (changing paths mess with sha1)
-              config.line_comments = false if ENV["TEST"]
+              compass_config.line_comments = false if ENV["TEST"]
 
-              if respond_to?(:asset_host) && asset_host.is_a?(Proc)
-                config.asset_host(&asset_host)
+              if config.defines_setting?(:asset_host) && config[:asset_host].is_a?(Proc)
+                compass_config.asset_host(&config[:asset_host])
               end
             end
 
diff --git a/middleman-more/lib/middleman-more/core_extensions/default_helpers.rb b/middleman-more/lib/middleman-more/core_extensions/default_helpers.rb
index b9b5076e..f9b66cb6 100644
--- a/middleman-more/lib/middleman-more/core_extensions/default_helpers.rb
+++ b/middleman-more/lib/middleman-more/core_extensions/default_helpers.rb
@@ -20,7 +20,7 @@ module Middleman
 
           app.helpers Helpers
 
-          app.set :relative_links, false
+          app.config.define_setting :relative_links, false, 'Whether to generate relative links instead of absolute ones'
         end
         alias :included :registered
       end
@@ -110,7 +110,7 @@ module Middleman
         # :relative option which, if set to true, will produce
         # relative URLs instead of absolute URLs. You can also add
         #
-        # set :relative_links, true
+        # config[:relative_links] = true
         #
         # to config.rb to have all links default to relative.
         def link_to(*args, &block)
@@ -136,10 +136,10 @@ module Middleman
 
               resource = sitemap.find_resource_by_path(url)
 
-              # Allow people to turn on relative paths for all links with set :relative_links, true
+              # Allow people to turn on relative paths for all links with config[:relative_links] = true
               # but still override on a case by case basis with the :relative parameter.
               effective_relative = relative || false
-              if relative.nil? && relative_links
+              if relative.nil? && config[:relative_links]
                 effective_relative = true
               end
 
diff --git a/middleman-more/lib/middleman-more/core_extensions/i18n.rb b/middleman-more/lib/middleman-more/core_extensions/i18n.rb
index eb23e70f..d0f6d80b 100644
--- a/middleman-more/lib/middleman-more/core_extensions/i18n.rb
+++ b/middleman-more/lib/middleman-more/core_extensions/i18n.rb
@@ -9,11 +9,11 @@ module Middleman
 
         # Once registerd
         def registered(app, options={})
-          app.set :locales_dir, "locales"
+          app.config.define_setting :locales_dir, "locales", 'The directory holding your locale configurations'
 
           # Needed for helpers as well
           app.after_configuration do
-            locales_glob = File.join(locales_dir, "*.yml");
+            locales_glob = File.join(config[:locales_dir], "*.yml");
 
             ::I18n.load_path += Dir[File.join(root, locales_glob)]
             ::I18n.reload!
diff --git a/middleman-more/lib/middleman-more/extensions/asset_host.rb b/middleman-more/lib/middleman-more/extensions/asset_host.rb
index 0cddce83..d43e9a39 100644
--- a/middleman-more/lib/middleman-more/extensions/asset_host.rb
+++ b/middleman-more/lib/middleman-more/extensions/asset_host.rb
@@ -11,7 +11,7 @@ module Middleman
         # Once registered
         def registered(app)
           # Default to no host
-          app.set :asset_host, false
+          app.config.define_setting :asset_host, false, 'The asset host to use, or false for no asset host, or a Proc to determine asset host'
 
           # Include methods
           app.send :include, InstanceMethods
@@ -30,12 +30,12 @@ module Middleman
         # @return [String]
         def asset_url(path, prefix="")
           original_output = super
-          return original_output unless asset_host
+          return original_output unless config[:asset_host]
 
-          asset_prefix = if asset_host.is_a?(Proc)
-            asset_host.call(original_output)
-          elsif asset_host.is_a?(String)
-            asset_host
+          asset_prefix = if config[:asset_host].is_a?(Proc)
+            config[:asset_host].call(original_output)
+          elsif config[:asset_host].is_a?(String)
+            config[:asset_host]
           end
 
           File.join(asset_prefix, original_output)
diff --git a/middleman-more/lib/middleman-more/extensions/minify_css.rb b/middleman-more/lib/middleman-more/extensions/minify_css.rb
index ac4b9644..d4f4bed0 100644
--- a/middleman-more/lib/middleman-more/extensions/minify_css.rb
+++ b/middleman-more/lib/middleman-more/extensions/minify_css.rb
@@ -10,13 +10,13 @@ module Middleman
 
         # Once registered
         def registered(app, options={})
-          app.set :css_compressor, false
+          app.config.define_setting :css_compressor, nil, 'Set the CSS compressor to use. Deprecated in favor of the :compressor option when activating :minify_css'
 
           ignore = Array(options[:ignore]) << /\.min\./
           inline = options[:inline] || false
 
           app.after_configuration do
-            chosen_compressor = css_compressor || options[:compressor] || begin
+            chosen_compressor = config[:css_compressor] || options[:compressor] || begin
               require "middleman-more/extensions/minify_css/rainpress"
               ::Rainpress
             end
diff --git a/middleman-more/lib/middleman-more/extensions/minify_javascript.rb b/middleman-more/lib/middleman-more/extensions/minify_javascript.rb
index 5ffae15c..9a1e91e6 100644
--- a/middleman-more/lib/middleman-more/extensions/minify_javascript.rb
+++ b/middleman-more/lib/middleman-more/extensions/minify_javascript.rb
@@ -10,7 +10,7 @@ module Middleman
 
         # Once registered
         def registered(app, options={})
-          app.set :js_compressor, false
+          app.config.define_setting :js_compressor, nil, 'Set the JS compressor to use. Deprecated in favor of the :compressor option when activating :minify_js'
 
           ignore = Array(options[:ignore]) << /\.min\./
           inline = options[:inline] || false