Issue running application

Hello,
When I run my MVC 3 application I get the following error
Could not load file or assembly 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified I've seen this issue before but sadly I can't remember the resolution for it, can someone shed some light on this please
Thanks