 <p>ah vb6 :) it's been a long time....</p> <p>basically, you can't debug .NET code within VB6 IDE.</p> <p>However, nothing stops you from creating a .NET test project to unit test the .NET dll. <em>And that's what you should have done prior to reference it in VB6.</em></p> <p>If you need to track down a specific issue, another way you can use is to write debugging infos to a file/database/event view/... when the dll is in debug mode, like which functions were called, parameters, stack trace... </p>