Simply set the Validator property true, and use as any other ASP.NET validator. When validation is performed on the server, if there are errors and the spell check hasn't already been performed then the validation warning will be shown. The user can leave spelling 'errors' in the text box (because some words, such as names, are not known by the spell checker, but are valid), but they must at least run the spell check if there are errors. Set the AutoLaunchWhenTextInvalid property to true to have the spell check run automatically after the page posts back - unlike the RapidSpellWebLauncher example, there are no popup blocker concerns with this behavior.