To reuse a problem-solving method, a knowledge engineer must know the assumptions about the structure of the knowledge roles inherent to the method. We construct a method ontology in order to formalize these assumptions. This ontology defines the roles played by the knowledge in the inference process, and serves as an interface for the knowledge engineer to evaluate if an application domain conforms to the structure of knowledge used by the method. In addition, we define the inferences of the problem-solving method according to its method ontology. This definition is declarative and abstracts from the implementation details. We illustrate our approach using the problem-solving method Propose&Revise. 1 INTRODUCTION Many approaches in knowledge engineering cope with the problem of constructing a library of reusable problem-solving methods (PSMs) (Chandrasekaran, 1983; Clancey, 1985; Steels, 1990; Musen et al., 1994; Breuker and Van de Velde, 1994). Here, we consider "problemsol...