Program understanding is the process of making sense of a complex source code. This process has been considered as computationally dificult and conceptually complex. So fal; no formal complexity results have been presented, and conceptual models differ from one researcher to the next.In this paper we formally prove that program understanding is NP-had. Furthermore, we show that even a much simpler subproblem remains NP-hard. Howevel; we do not despair by this result, but rathel; offer an attractive problem-solving model for the program understanding problem. Our model is built on a framework for solving Constraint Satisfaction Problems, or CSPs, which are known to have interesting heuristic solutions. Specifically, we can represent and heuristically address previous and new heuristic approaches to the program understanding problem with both existing and specially designed constraint propagation and search algorithms.