The evolutionary case approach provides a framework for qualitative case study research in information systems (IS). It uses revelation, reinforcement, reflection and re-examination to explicitly explore emerging themes in interpretive case study research. The method is based on the progressive development of a theoretical model grounded initially in the literature and then refined using sequential case studies grounded in practice. The method addresses the gap which often separates data from conclusions in qualitative case study research by documenting the “revealed” and “reinforced” changes in the theoretical model as it evolves from the empirical data. The paper provides an illustrative study of the use of models in object-oriented requirements engineering to demonstrate the use of the evolutionary case approach.