In this article, we introduce the structure, possibilities and limitations of the first version of the microsimulation model MEFISTO (Modelling and Evaluating Flanders’ FIscal and Social TOmorrow). MEFISTO is a web-enabled microsimulation model for Flanders that is publicly available on www.flemosi.be. We compare MEFISTO to alternative models for ex ante policy evaluation in Flanders. We illustrate its use and user-friendliness by showing how policy reforms can easily be implemented and how the results are to be interpreted. We end the article by previewing the main directions in which the next versions of the model will be developed.