.NET Extension Methods - BC30002: Type 'Extension' is not defined.

I was trying to add an extension module to a project and my <Extension()> declaration said it was undefined.  Found out I needed to include System.Core in my Assemblies in my Web.Config

BC30002: Type 'Extension' is not defined.

<add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>