ViewState is a client-side state management mechanism in ASP.NET. It is a default technique used by ASP.NET to persist the value of the page and controls during postbacks. In ASP.NET ViewState the values are encrypted and stored in a hidden field ( named _VIEWSTATE) on the page as an encoded Base64 string. By default, ViewState is sent to the client browser and then returned to the server in the form of a hidden input control on your page.