I found this "If you look at the plugin tutorials you’ll notice that IModule only has inline and pure virtual methods, and its parent class IModuleInterface likewise only contains inline methods, there’s no need to export anything in order to use either of those classes in your project because all the relevant information is already in the header files. The same thing applies to UE modules that don’t use MODULE_API. In your case however, your FTestModule has a Test method that isn’t inline, nor pure virtual, which means that when you link your project against your module the linker is going to try and find the definition of that method in your module’s library. You can read more about the different ways to export classes from DLLs in this article. " in link text