In this project, we transferred Problem-Based Learning (PBL), a well-known pedagogy for medical education, to an online environment by transforming it into a simulation where students could practice their skills collaboratively in a risk-free setting. This paper describes the design of a conceptual model and development of a software platform to allow learners to collaborate online in discussing PBL cases. Early testing has been extremely promising, largely due to the extensive work done earlier to build the ENJEUX-S online game platform. Thanks to this work, it has been possible to extend this platform from online games to online simulations. Our first test of the PBL model with undergraduate students was successful in facilitating their critical thinking and was well-received. We are now planning to use COMPSoft in medical schools and to conduct comprehensive evaluation studies of both the model and the platform.