diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index a555e01..681c64a 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -811,7 +811,7 @@ module Oga
       # Processes the `starts-with()` function call.
       #
       # This function call returns `true` if the string in the 1st argument
-      # starts with the string in the 2nd argument.
+      # starts with the string in the 2nd argument. Node sets can also be used.
       #
       # @example
       #  starts-with("hello world", "hello") # => true
@@ -828,6 +828,22 @@ module Oga
         return haystack_str.start_with?(needle_str)
       end
 
+      ##
+      # Processes the `contains()` function call.
+      #
+      # This function call returns `true` if the string in the 1st argument
+      # contains the string in the 2nd argument. Node sets can also be used.
+      #
+      # @example
+      #  contains("hello world", "o w") # => true
+      #
+      def on_call_contains(context, haystack, needle)
+        haystack_str = on_call_string(context, haystack)
+        needle_str   = on_call_string(context, needle)
+
+        return haystack_str.include?(needle_str)
+      end
+
       ##
       # Processes an `(int)` node.
       #
diff --git a/spec/oga/xpath/evaluator/calls/contains_spec.rb b/spec/oga/xpath/evaluator/calls/contains_spec.rb
new file mode 100644
index 0000000..3725ed4
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/contains_spec.rb
@@ -0,0 +1,38 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'contains() function' do
+    before do
+      @document  = parse('<root><a>foo</a><b>foobar</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return true if the 1st string contains the 2nd string' do
+      @evaluator.evaluate('contains("foobar", "oo")').should == true
+    end
+
+    example "return false if the 1st string doesn't contain the 2nd string" do
+      @evaluator.evaluate('contains("foobar", "baz")').should == false
+    end
+
+    example 'return true if the 1st node set contains the 2nd string' do
+      @evaluator.evaluate('contains(root/a, "oo")').should == true
+    end
+
+    example 'return true if the 1st node set contains the 2nd node set' do
+      @evaluator.evaluate('contains(root/b, root/a)').should == true
+    end
+
+    example "return false if the 1st node doesn't contain the 2nd node set" do
+      @evaluator.evaluate('contains(root/a, root/b)').should == false
+    end
+
+    example 'return true if the 1st string contains the 2nd node set' do
+      @evaluator.evaluate('contains("foobar", root/a)').should == true
+    end
+
+    example 'return true when using two empty strings' do
+      @evaluator.evaluate('contains("", "")').should == true
+    end
+  end
+end