Mental models play an important role in science education research. To extend the effectiveness of conceptual change research and to improve mental model identi?cation and diagnosis, the authors developed and tested the Web-Based Mental Models Diagnosis (WMMD) system. In this article, they describe their WMMD system, which goes beyond the capabilities of the existing technology-based assessment tools to investigate whether learners develop and possess single or multiple mental models while learning science. (Contains 3 figures.)