This paper introduces a generalised model building platform (MMEANS) for implementing and using tax-benefit microsimulation models. It is designed to aid in the construction of single- and multi-country tax-benefit models by providing all essential components and a system by which these can be parameterised and combined into a full model. We explain the conceptual and computational issues arising in the design and development of MMEANS. One application of the software has been to construct EUROMOD, a 15 country European tax-benefit model (Immervoll et al., 1999; Sutherland, 2001). However, we argue that, apart from its direct usefulness for this model, MMEANS can be used as a general software tool for microsimulation model building.