In this paper we introduce a perspective on multidisciplinary design optimization (MDO) problem formulation that provides a basis for choosing among existing formulations and suggests provocative, new ones. MDO problems offer a richer spectrum of possibilities for problem formulation than do single discipline design optimization problems, or multidisciplinary analysis problems. This is because the variables and the equations that characterize the MDO problem can be "partitioned" in some interesting ways between what we traditionally think of as the "analysis code(s)" and the "optimization code." An MDO approach can be characterized by what part of the overall computation is done in each code, how that computation is done, and what information is communicated between the codes.