Client side form validation vs server side form validation?

Best Practice : Use both The Cons of using one or the other : With client side validation only, if a user disables JavaScript, then no validation will occur. With server side validation only, there are more return trips to the server which in turns uses more resources and slows down the site when a client side script could have done the validation without the server. The Pros of having both : The data will always be validated, whether or not the client has JavaScript enabled or not. There will be less server strain with the client side validation. The client can fix form errors faster with instant client side notifications. The data will have a bit more integrity going through the dual validation process.