In the previous version of asp.net, v1.1, the CustomValidator did NOT run when the control(e.g TextBox) was empty. Possible solutions were to create your own control (something like RequiredCustomValidator : CustomValidator) or the change WebUIValidation.js file so it will actually validate the control even if it's empty. Now, in the new version, v2.0, you can simply set the ValidateEmptyText to true and you're done. Nice (and useful) !