diff --git a/lib/jekyll.rb b/lib/jekyll.rb
index 03724100..0a7c2b69 100644
--- a/lib/jekyll.rb
+++ b/lib/jekyll.rb
@@ -48,6 +48,7 @@ module Jekyll
   autoload :Layout,              'jekyll/layout'
   autoload :LayoutReader,        'jekyll/layout_reader'
   autoload :LogAdapter,          'jekyll/log_adapter'
+  autoload :Metadata,            'jekyll/metadata'
   autoload :Page,                'jekyll/page'
   autoload :PluginManager,       'jekyll/plugin_manager'
   autoload :Post,                'jekyll/post'
diff --git a/lib/jekyll/cleaner.rb b/lib/jekyll/cleaner.rb
index 6dd59ea0..79f97f47 100644
--- a/lib/jekyll/cleaner.rb
+++ b/lib/jekyll/cleaner.rb
@@ -21,7 +21,14 @@ module Jekyll
       #
       # Returns an Array of the file and directory paths
       def obsolete_files
-        (existing_files - new_files - new_dirs + replaced_files).to_a
+        (existing_files - new_files - new_dirs + replaced_files + metadata_file).to_a
+      end
+
+      # Private: The metadata file storing dependency tree and build history
+      #
+      # Returns an Array with the metdata file as the only item
+      def metadata_file
+        [site.metadata.metadata_file]
       end
 
       # Private: The list of existing files, apart from those included in keep_files and hidden files.
diff --git a/lib/jekyll/command.rb b/lib/jekyll/command.rb
index a1cc8c01..d463d235 100644
--- a/lib/jekyll/command.rb
+++ b/lib/jekyll/command.rb
@@ -58,6 +58,7 @@ module Jekyll
         c.option 'unpublished', '--unpublished', 'Render posts that were marked as unpublished'
         c.option 'quiet',   '-q', '--quiet', 'Silence output.'
         c.option 'verbose', '-V', '--verbose', 'Print verbose output.'
+        c.option 'clean', '-c', '--clean', 'Clean the site before rebuilding.'
       end
 
     end
diff --git a/lib/jekyll/convertible.rb b/lib/jekyll/convertible.rb
index 4b21a66b..4b1f3761 100644
--- a/lib/jekyll/convertible.rb
+++ b/lib/jekyll/convertible.rb
@@ -207,6 +207,12 @@ module Jekyll
                                          info,
                                          File.join(site.config['layouts'], layout.name))
 
+        # Add layout to dependency tree
+        site.metadata.add_dependency(
+          Jekyll.sanitized_path(site.source, path),
+          Jekyll.sanitized_path(site.source, layout.path)
+        )
+
         if layout = layouts[layout.data["layout"]]
           if used.include?(layout)
             layout = nil # avoid recursive chain
diff --git a/lib/jekyll/layout.rb b/lib/jekyll/layout.rb
index 4dde59b6..f973019f 100644
--- a/lib/jekyll/layout.rb
+++ b/lib/jekyll/layout.rb
@@ -8,6 +8,9 @@ module Jekyll
     # Gets the name of this layout.
     attr_reader :name
 
+    # Gets the path to this layout.
+    attr_reader :path
+
     # Gets/Sets the extension of this layout.
     attr_accessor :ext
 
@@ -26,6 +29,7 @@ module Jekyll
       @site = site
       @base = base
       @name = name
+      @path = Jekyll.sanitized_path(site.source, File.join(base, name))
 
       self.data = {}
 
diff --git a/lib/jekyll/metadata.rb b/lib/jekyll/metadata.rb
new file mode 100644
index 00000000..abda35e5
--- /dev/null
+++ b/lib/jekyll/metadata.rb
@@ -0,0 +1,87 @@
+require 'set'
+
+module Jekyll
+  class Metadata
+    attr_reader :site, :metadata
+
+    def initialize(site)
+      @site = site
+
+      # Initialize metadata store by reading YAML file,
+      # or an empty hash if file does not exist
+      @metadata = (File.file?(metadata_file) && !(site.config['clean'])) ? SafeYAML.load(File.read(metadata_file)) : {}
+
+      # Initialize cache to an empty hash
+      @cache = {}
+    end
+
+    # Add a path to the metadata
+    #
+    # Returns true.
+    def add(path)
+      @metadata[path] = {
+        "mtime" => File.mtime(path),
+        "deps" => []
+      }
+      @cache[path] = true
+    end
+
+    # Force a path to regenerate
+    #
+    # Returns true.
+    def force(path)
+      @cache[path] = true
+    end
+
+    # Checks if a path should be regenerated
+    #
+    # Returns a boolean.
+    def regenerate?(path)
+      # Check for path in cache
+      if @cache.has_key? path
+        return @cache[path]
+      end
+
+      # Check path that exists in metadata
+      if (data = @metadata[path])
+        data["deps"].each do |dependency|
+          if regenerate?(dependency)
+            return @cache[dependency] = @cache[path] = true
+          end
+        end
+        if data["mtime"] == File.mtime(path)
+          return @cache[path] = false
+        else
+          return add(path)
+        end
+      end
+
+      # Path does not exist in metadata, add it
+      return add(path)
+    end
+
+    # Add a dependency of a path
+    #
+    # Returns true.
+    def add_dependency(path, dependency)
+      @metadata[path]["deps"] << dependency unless @metadata[path]["deps"].include? dependency
+      add(dependency)
+    end
+
+    # Write the metadata to disk
+    #
+    # Returns nothing.
+    def write
+      File.open(metadata_file, 'w') do |f|
+        f.write(@metadata.to_yaml)
+      end
+    end
+
+    # Produce the absolute path of the metadata file
+    #
+    # Returns the String path of the file.
+    def metadata_file
+      Jekyll.sanitized_path(site.source, '.jekyll-metadata')
+    end
+  end
+end
diff --git a/lib/jekyll/renderer.rb b/lib/jekyll/renderer.rb
index f88a4187..61839353 100644
--- a/lib/jekyll/renderer.rb
+++ b/lib/jekyll/renderer.rb
@@ -138,6 +138,12 @@ module Jekyll
           File.join(site.config['layouts'], layout.name)
         )
 
+        # Add layout to dependency tree
+        site.metadata.add_dependency(
+          Jekyll.sanitized_path(site.source, document.path),
+          Jekyll.sanitized_path(site.source, layout.path)
+        )
+
         if layout = site.layouts[layout.data["layout"]]
           if used.include?(layout)
             layout = nil # avoid recursive chain
diff --git a/lib/jekyll/site.rb b/lib/jekyll/site.rb
index 115b4133..38c61661 100644
--- a/lib/jekyll/site.rb
+++ b/lib/jekyll/site.rb
@@ -11,6 +11,7 @@ module Jekyll
                   :gems, :plugin_manager
 
     attr_accessor :converters, :generators
+    attr_accessor :metadata
 
     # Public: Initialize a new Site.
     #
@@ -27,6 +28,9 @@ module Jekyll
       @source              = File.expand_path(config['source']).freeze
       @dest                = File.expand_path(config['destination']).freeze
 
+      # Build metadata
+      @metadata = Metadata.new(self)
+
       self.plugin_manager = Jekyll::PluginManager.new(self)
       self.plugins        = plugin_manager.plugins_path
 
@@ -49,7 +53,7 @@ module Jekyll
       read
       generate
       render
-      cleanup
+      cleanup if config['clean']
       write
     end
 
@@ -289,13 +293,17 @@ module Jekyll
 
       collections.each do |label, collection|
         collection.docs.each do |document|
-          document.output = Jekyll::Renderer.new(self, document).run
+          if @metadata.regenerate?(document.path)
+            document.output = Jekyll::Renderer.new(self, document).run
+          end
         end
       end
 
       payload = site_payload
       [posts, pages].flatten.each do |page_or_post|
-        page_or_post.render(layouts, payload)
+        if @metadata.regenerate?(Jekyll.sanitized_path(source, page_or_post.relative_path))
+          page_or_post.render(layouts, payload)
+        end
       end
     rescue Errno::ENOENT => e
       # ignore missing layout dir
@@ -312,7 +320,12 @@ module Jekyll
     #
     # Returns nothing.
     def write
-      each_site_file { |item| item.write(dest) }
+      each_site_file { |item|
+        if @metadata.regenerate? Jekyll.sanitized_path(source, item.path)
+          item.write(dest)
+        end
+      }
+      @metadata.write
     end
 
     # Construct a Hash of Posts indexed by the specified Post attribute.
diff --git a/lib/jekyll/tags/include.rb b/lib/jekyll/tags/include.rb
index 3eb4d7c0..8f7da2ca 100644
--- a/lib/jekyll/tags/include.rb
+++ b/lib/jekyll/tags/include.rb
@@ -105,13 +105,20 @@ eos
       end
 
       def render(context)
+        site = context.registers[:site]
         dir = resolved_includes_dir(context)
 
         file = render_variable(context) || @file
         validate_file_name(file)
 
         path = File.join(dir, file)
-        validate_path(path, dir, context.registers[:site].safe)
+        validate_path(path, dir, site.safe)
+
+        # Add include to dependency tree
+        site.metadata.add_dependency(
+          Jekyll.sanitized_path(site.source, context.registers[:page]["path"]),
+          path
+        )
 
         begin
           partial = Liquid::Template.parse(source(path, context))
diff --git a/site/.gitignore b/site/.gitignore
index 79bd74f7..c1bd964c 100644
--- a/site/.gitignore
+++ b/site/.gitignore
@@ -2,3 +2,4 @@ _site/
 *.swp
 pkg/
 test/
+.jekyll-metadata