I'm trying to use Ajax Control Toolkit's Autocomplete extender with a WCF Service. This question is almost what I'm looking for, one of the answers points to a tutorial but I can't get it to work. In my solution I have a web form application project and a WCF service library project. One of the properties of the Autocomplete extender is ServicePath which the tutorial points to a svc file: <form id="form1" runat="server"> <asp:ScriptManager ID="ScriptManager1" runat="server"> </asp:ScriptManager> <div> <asp:TextBox runat="server" ID="myTextBox" Width="300" autocomplete="off" /> <ajaxToolkit:AutoCompleteExtender runat="server" BehaviorID="AutoCompleteEx" ID="autoComplete1" TargetControlID="myTextBox" ServicePath="Autocomplete.svc" ServiceMethod="GetCompletionList" MinimumPrefixLength="0" CompletionInterval="1000" EnableCaching="true"> </ajaxToolkit:AutoCompleteExtender> </div> </form> Right now in ServicePath I'm pointing to the http address (http://localhost:8731/Design_Time_Addresses/WebApp.WcfServiceLibrary/ProductService/) that my WCF Service is running, but it just don't works.