Web Site projects were introduced with VS 2005. Many people complained because the VS 2003 manner of creating compiled Web Applications were more efficient and easy to manage. Here is an article that explains the Web Application project which became part of SP1… It is much more highly scalable… http://msdn2.microsoft.com/en-us/library/Aa730880(VS.80).aspx Following is the article that stats steps to convert a web site project to web application project. http://webproject.scottgu.com/CSharp/Migration2/Migration2.aspx During migration, it will be changes in the project structure and additional files will be created (.designer). App_code files will be moved to Old_App_Code folder and there will be one assembly file for complete project. One assembly will restrict our flexibility to use C# code along with VB.NET because .NET cannot compile multiple languages into one assembly.