I am writing an ASP.NET MVC 2 application using NHibernate and repository pattern. I have an assembly that contains my model (business entities), moreover in my web project I want to use flattened objects (possibly with additional properties/logic) as ViewModels. These VMs contain UI-specific metadata (eg. DisplayAttribute used by Html.LabelFor() method). The problem is that I don't know how to implement validation so that I don't repeat myself throughout various tiers (specifically validation rules are written once in Model and propagated to ViewModel). I am using DataAnnotations on my ViewModel but this means no validation rules are imposed on the Model itself. One approach I am considering is deriving ViewModel objects from business entities adding new properties/overriding old ones, thus preserving validation metadata between the two however this is an ugly workaround. I have seen Automapper project which helps to map properties, but I am not sure if it can handle ASP.NET MVC 2 validation metadata properly. Is it difficult to use custom validation framework in asp.net mvc 2? Do you have any patterns that help to preserve DRY in regard to validation?