When we use the AJAX Control Toolkit ModalPopupExtender, sometimes we may get below error Sys.Extended is undefined$create(Sys.Extended.UI.ModalPopupBehavior, {"BackgroundCssClass":... After few hours of investigation, I realized that keyword I’m using it for scriptmanager.If you are using .net 4.0 framework, then change it from <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager> To <asp:scriptmanager  ID="ToolkitScriptManager1" runat="server"></asp:scriptmanager > And vice-versa for 3.5 and below version.