IT

When building AJAX-enabled web pages with the ASP.NET AJAX framework, you must add precisely one ScriptManager control to each and every page that uses the framework.[1]

As its name implies, the ScriptManager manages the client-side script used in AJAX-enabled web pages. At a minimum, the ScriptManager emits HTML that instructs the browser to download the JavaScript files that makeup the ASP.NET AJAX Client Library. It can also be used to register custom JavaScript files, script-enabled web services, and custom application service functionality.[1]

Every web page that uses the ASP.NET AJAX framework must contain precisely one ScriptManager control. Because of this requirement, it usually makes sense to place a single ScriptManager control on the master page so that all content pages have the ScriptManager control automatically included. Furthermore, the ScriptManager must come before any of the ASP.NET AJAX server controls, such as the UpdatePanel and UpdateProgress controls. Therefore, it's best to put the ScriptManager before any ContentPlaceHolder controls within the Web Form.[1]

See also:
ScriptManagerProxy

Related articles:
[1] - Master Pages and ASP.NET AJAX: This tutorial shows how to add the ScriptManager control to the master page. It also looks at how to use the ScriptManagerProxy control to register custom scripts and script services in a specific content page.