What kinds of validation are appropriately implemented on the client and what kinds are more appropriately implemented on the server. I am using ASP.NET validation controls and I am not sure whether I should enable/disable client side validation

Thank you

It is important that you do not think client-side validation and server-side validation is a one-or-the-other choice.

Client-side validation is a nicety for the benefit of the user. He or she will get immediate feedback if some data is invalid. If it were not for client-side validation, the user would have to wait while the page posted back, and then returned with its validation messages.

Server-side validation is a necessity for the benefit of your application. Even if client-side validation is in place, you must back that up with server-side validation of the same data. It is very easy for a user to intentionally or unintentionally side-step client-side validation. Knowing that, server-side validation becomes a necessity.

So always use server-side validation. Then, if possible, add client-side validation to improve the user experience. Server-side validation is required; client-side validation is optional.
 

Client side validation should never be used exclusively, as it is too easy to sidestep (the user just has to disable javascript in his/her browser f.ex). It is great for creating a better, visibly more intuitive user experience by popping up informative error messages next to input fields etc in 'realtime', but you should always have server-side validation to do the 'real' validation after the user has submitted the form. The ASP.NET Validation controls let you do exactly this :)

A few suggestions:
1. If you create your own validator (including a CustomValidator), always make sure it has server side validation code. The client-side code is optional. If omitted, set the EnableClientScript property to false.
2. In your Click event method, always test Page.IsValid is true before attempting to save/continue.

Please see this article for more details including a lot more knowledge on ASP.NET validators: www.aspalliance.com/699.