This paper gives an overview of some of the modelling and virtual prototyping techniques used in product realization, with emphasis on the mechanical engineering field. It is pointed out that virtual prototypes, in the commonly accepted sense of computer models permitting realistic graphical simulation, represent only one class amongst the many types of computer models used in design and planning for manufacture. Each such model is usually created for some comparatively narrow purpose, and one of the major problems faced by developers of integrated computer-aided product realization systems concerns the transmutation of one type of model into another. A related problem is that of interpretation by any model of information generated by interrogations of another model. These difficulites are compounded by the increasing presence in such models of semantic information concerning different aspects of the intended functionality or manufacturing requirements of the modelled artifact.