A Marshal/PInvoke developer specializes in bridging managed and unmanaged code in .NET applications. They use Platform Invocation Services (PInvoke) to call C-style functions in native dynamic link libraries (DLLs) from managed code. They also handle marshalling, the process of transforming types as they cross the managed-unmanaged boundary, ensuring data types match between .NET and the DLL. Their tasks involve understanding both managed and unmanaged environments, identifying compatibility issues, and effectively using interop services to maintain performance and memory management. They contribute significantly to integrating existing legacy libraries or OS-specific features into .NET applications.