Model dialog boxes are very common these days not only in Windows applications but also in web applications. They allow web developers to open new popup windows on the same page without creating a new web page. These popup windows typically ask user some input and temporarily disable the page unless the user dismisses the dialog box. ASP.NET developers can use ModelPopupExtender control available in ASP.NET AJAX controls toolkit to implement model dialog behavior in their web applications. In this tutorial, I will show you how you can use ModelPopupExtender control with client side behavior so that when user performs any action in the dialog box such as button click there is no post back to the server.