[How Do I:] Use the ASP.NET AJAX History Control?
One problem faced by a typical AJAX application is that a partial page update is not added to the history of the Web browser. This means that the browser's Back button does not move back one AJAX step, but moves back one entire document, which is unlikely to be what the user expects. The ASP.NET AJAX History control allows the developer to insert history points so that the user may click the browser's Back and Forward buttons to move between AJAX steps.