diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index 9d1b6df..03c6ba2 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -966,6 +966,22 @@ module Oga
         return haystack_str[start_index..stop_index]
       end
 
+      ##
+      # Processes the `string-length()` function.
+      #
+      # This function returns the length of the string given in the 1st argument
+      # *or* the current context node. If the expression is not a string it's
+      # converted to a string using the `string()` function.
+      #
+      # @see [#on_call_string]
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Float]
+      #
+      def on_call_string_length(context, expression = nil)
+        return on_call_string(context, expression).length.to_f
+      end
+
       ##
       # Processes an `(int)` node.
       #
diff --git a/spec/oga/xpath/evaluator/calls/string_length_spec.rb b/spec/oga/xpath/evaluator/calls/string_length_spec.rb
new file mode 100644
index 0000000..1a63da6
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/string_length_spec.rb
@@ -0,0 +1,42 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'string-length() function' do
+    before do
+      @document  = parse('<root><a>x</a></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'outside predicates' do
+      example 'return the length of a literal string' do
+        @evaluator.evaluate('string-length("foo")').should == 3.0
+      end
+
+      example 'return the length of a literal integer' do
+        @evaluator.evaluate('string-length(10)').should == 2.0
+      end
+
+      example 'return the length of a literal float' do
+        # This includes the counting of the dot. That is, "10.5".length => 4
+        @evaluator.evaluate('string-length(10.5)').should == 4.0
+      end
+
+      example 'return the length of a string in a node set' do
+        @evaluator.evaluate('string-length(root)').should == 1.0
+      end
+    end
+
+    context 'inside predicates' do
+      before do
+        # Since the length of <a> is 1 this should just return the <a> node.
+        @set = @evaluator.evaluate('root/a[string-length()]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <a> node' do
+        @set[0].should == @document.children[0].children[0]
+      end
+    end
+  end
+end