Visual Studio 2012 SPA Template /Help/index.cshtml file was throwing your error.
@using System.Web.Http @using System.Web.Http.Description <=== CS0234: The type or namespace name ‘Description‘ does not exist in the namespace ‘System.Web.Http‘ (are you missing an assembly reference?)
Solution: Set "Copy Local : True" in properties for References\System.Web.Http
http://stackoverflow.com/questions/12271431/system-web-http-description-is-missing-from-add-reference-list
The type or namespace name 'Description' does not exist in the namespace 'System.Web.Http'
原文：http://www.cnblogs.com/cn-chenhao/p/4673379.html