I remember seeing a sample which used Sys.Application.add_load to attach a function you’d normally want to run upon window.onload. It went like this: function initFoo () { // Initialization code here... } Sys.Application.add_load (initFoo); Having browsed the MS AJAX Library source code, I figured the Sys.Application provided a convenient event hook indeed since the class tapped window.load and window.unload for you: Sys.UI.DomEvent.addHandler (window, "unload", this._unloadHandlerDelegate); Sys.UI.DomEvent.addHandler (window, "load", this._loadHandlerDelegate); To my surprize, initFoo was getting called on every AJAX callback. I figured there was no way get window to fire its onload event several times, so the Sys.Application class must’ve been involved a lot more than I thought. The remedy is to either hook into window.onload directly: Sys.UI.DomEvent.addHandler(window, "load", initFoo); or avoid multiple initializations: function initFoo () { var prm =