Interop Assembly

Question

What is the correct procedure to include/refer to an interop assembly in a .NET project? Here I have an assembly (dll) from the bin of a Windows Application (C#.NET) which I would like to include in an ASP.NET_C# application. I have added reference to the same in the project but upon build (F5), I get an Error in the Task List that I might have missed a using directive or an assembly reference. I have also included the assembly in the Code-Behind of the page with the 'using' keyword but the system still gives the same error. Your suggestions are most valuable.