We present a system (ASP − PROLOG) which provides a tight and well-defined integration of Prolog and Answer Set Programming (ASP). The combined system enhances the expressive power of ASP, allowing us to write programs that reason about dynamic ASP modules and about collections of stable models. These features are vital in a number of application domains (e.g., planning, scheduling, diagnosis). We describe the design of ASP − PROLOG along with its implementation, realized using CIAO Prolog and Smodels.