Combining various knowledge types - and reasoning methods - in knowledge-based systems is a challenge to the knowledge representation task. The paper describes an object-oriented, frame-based knowledge representation system aimed at unifying case-specific and general domain knowledge within a single representation system. It is targeted at the representational needs that have emerged from research in knowledge-intensive casebased reasoning, addressing complex problem solving in open and weak theory domains. Emphasis is put on representational expressiveness, on flexible reasoning and control schemes, and on easy inspection of cases and other knowledge objects.