Modelling complex software systems requires multiple modelling formalisms adapted to the nature of each part of the system (control, signal processing, etc.), to the aspect on which the model focuses (functionality, time, fault tolerance, etc.) and to the level of abstraction at which the system, or one of its parts, is studied. The use of different modelling formalisms during the development cycle is therefore both unavoidable and essential. As a consequence, system designers deal with a large variety of models that relate to a given system but do not form a global model of this system. A major difficulty is then to answer questions about properties of the whole system, and in particular about its behaviour. Multi-Formalism Modelling allows the joint use of different modelling formalisms in a given model to overcome issues related to the integration of heterogeneous models. It applies to different tasks of the development cycle such as simulation, verification or testing.1 We propose an approach to multi-formalism modelling, called ModHel'X, which is based on the concept of Model of Computation and focuses on the simulation of models. Our approach addresses two important issues in this particular field: (a) providing support for the specification of the execution semantics of a modelling formalism, and (b) allowing the specification of the interactions between parts of a model described using different modelling formalisms.