PostBack is a process of submitting an ASP.NET page to the server for processing. PostBack is done if certain credentials of the page are to be checked against some sources such as verification of username and password using database. This is something that a client machine is not able to accomplish and thus these details have to be PostBack to the server. PostBack is actually sending all the information from client to web server, then web server process all those contents and returns back to the client. Most of the time ASP control will cause a PostBack but some don't unless you tell them to do In certain events Listbox Index Changed,RadioButton Checked etc. In an ASP.NET page upon which aPostBack might be needed. Is PostBack is a property of the Asp.Net page that tells whether or not the page is on its initial load or if a user has perform a button on your web page that has caused the page to post back to itself. The value of the Page.Is PostBack property will be set to true when the page is executing after a PostBack and false otherwise. We can check the value of this property based on the value and we can populate the controls on the page.