I had a VB.NET project which referenced a specific DLL that came from another source (I didn't have the code just the dll file). I added the dll to my project by browsing to the dll. The problem occurred when I tried to rebuild the project. All of a sudden I got a reference not found, and when I went to the directory where the dll was located it was no longer there. The solution was simple one. When adding the dll, in the Reference Properties window, you need to specify the "Specific Version" field to "True" instead of the default which is "False". By doing this you don't delete the dll.