Abstract: : Program debugging is a time consuming process. Conventional debugging techniques and aids typically give the user a narrow view of the program's operation, making debugging difficult. A debugging system that would present a clear overall picture of a program's behavior and would be both flexible and simple to operate would be a valuable tool. Such a system was designed and implemented in and for MDL, a high-level applicative programming language. This report discusses: the design alternatives considered during the debugging system's design and implementation phases, the reasons for the resulting design choices, and the system attributes. A major attribute of the system (MEND) is that it does not simulate the program being debugged but instead monitors it from another process. This attribute results in a robust and viable debugging system, because MEND need not be modified in order to handle each new extension to MDL and/or each new user-defined primitive. (Author)