diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index df1eef7..12f871f 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -99,6 +99,8 @@ module Oga
       def on_absolute_path(ast_node, context)
         if @document.respond_to?(:root_node)
           context = XML::NodeSet.new([@document.root_node])
+        else
+          context = XML::NodeSet.new([@document])
         end
 
         return on_path(ast_node, context)
@@ -651,6 +653,46 @@ module Oga
         return retval.length.to_f
       end
 
+      ##
+      # Processes the `id()` function call.
+      #
+      # The XPath specification states that this function's behaviour should be
+      # controlled by a DTD. If a DTD were to specify that the ID attribute for
+      # a certain element would be "foo" then this function should use said
+      # attribute.
+      #
+      # Oga does not support DTD parsing/evaluation and as such always uses the
+      # "id" attribute.
+      #
+      # This function searches the entire document for a matching node,
+      # regardless of the current position.
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_call_id(context, expression)
+        id    = process(expression, context)
+        nodes = XML::NodeSet.new
+
+        # Based on Nokogiri's/libxml behaviour it appears that when using a node
+        # set the text of the set is used as the ID.
+        id  = id.is_a?(XML::NodeSet) ? id.text : id.to_s
+        ids = id.split(' ')
+
+        @document.each_node do |node|
+          next unless node.is_a?(XML::Element)
+
+          attr = node.attribute('id')
+
+          if attr and ids.include?(attr.value)
+            nodes << node
+          end
+        end
+
+        return nodes
+      end
+
       ##
       # Processes an `(int)` node. This method simply returns the value as a
       # Float.
@@ -663,6 +705,17 @@ module Oga
         return ast_node.children[0].to_f
       end
 
+      ##
+      # Processes a `(string)` node.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [String]
+      #
+      def on_string(ast_node, context)
+        return ast_node.children[0]
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.
diff --git a/spec/oga/xpath/evaluator/calls/id_spec.rb b/spec/oga/xpath/evaluator/calls/id_spec.rb
new file mode 100644
index 0000000..7f17b75
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/id_spec.rb
@@ -0,0 +1,52 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'id() function' do
+    before do
+      @document  = parse('<root id="r1"><a id="a1"></a><a id="a2">a1</a></root>')
+      @first_a   = @document.children[0].children[0]
+      @second_a  = @document.children[0].children[1]
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'using a single string ID' do
+      before do
+        @set = @evaluator.evaluate('id("a1")')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the first <a> node' do
+        @set[0].should == @first_a
+      end
+    end
+
+    context 'using multiple string IDs' do
+      before do
+        @set = @evaluator.evaluate('id("a1 a2")')
+      end
+
+      it_behaves_like :node_set, :length => 2
+
+      example 'return the first <a> node' do
+        @set[0].should == @first_a
+      end
+
+      example 'return the second <a> node' do
+        @set[1].should == @second_a
+      end
+    end
+
+    context 'using a node set' do
+      before do
+        @set = @evaluator.evaluate('id(root/a[2])')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the first <a> node' do
+        @set[0].should == @first_a
+      end
+    end
+  end
+end
diff --git a/spec/oga/xpath/evaluator/types/string_spec.rb b/spec/oga/xpath/evaluator/types/string_spec.rb
new file mode 100644
index 0000000..f310350
--- /dev/null
+++ b/spec/oga/xpath/evaluator/types/string_spec.rb
@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'string types' do
+    before do
+      document  = parse('<a></a>')
+      evaluator = described_class.new(document)
+      @string   = evaluator.evaluate('"foo"')
+    end
+
+    example 'return the literal string' do
+      @string.should == 'foo'
+    end
+  end
+end