ScottGu: ASP.NET MVC 2: Model Validation

"Validating user-input and enforcing business rules/logic is a core requirement of most web applications. ASP.NET MVC 2 includes a bunch of new features that make validating user input and enforcing validation logic on models/viewmodels significantly easier. These features are designed so that the validation logic is always enforced on the server, and can optionally also be enforced on the client via JavaScript"