A message-passing library interface like MPI or PVM is only one interface between only two components the complete environment seen be the user of a parallel system. Here we discuss a larger set of components and draw attention to the usefulness of considering them separately. Such an approach causes us to focus on the interfaces among such components. Our primary motivation is the efficient use of large clusters to run MPI programs, and we describe current efforts by our group at Argonne to address some of the interface issues that arise in this context.