Modelling is an extremely important aspect of the work of engineers. Ever since technology changed from a craft-based to a science-based practice, engineers have been engaged in modelling the artefacts they design, build and test. The modelling techniques they rely on, however, originate from the physical sciences. They work well for the technical devices, modelled as physical systems, that are the traditional products of the engineering disciplines. It is increasingly recognized, however, that modern technology consists in the implementation and operation of systems rather than single devices. The traditional conceptual framework of engineering, derived from the natural sciences, is ill-fit to model the hybridity and mereological complexity that are the key features of systems in technology. In this paper I present an approach to the modelling of systems in technology which is based on an incorporation of these two aspects from the start, represented in the notion of an instrumental system. I first show how the hybridity—the interaction between intentional action and causal processes—is taken care of in the basic structure of any instrumental system. Next I show how the representation of mereological complexity is taken care of through recursion. Finally relevance and potential applications of the approach are discussed.