The Ptolemy project studies modelling, simulation and design of concurrent, real-time, embedded systems. The focus is on assembly of concurrent components. The key underlying principle in that project is the use of well-defined models of computation that govern the interaction between components. A major problem area being addressed is the use of heterogeneous mixtures of models of computation. A software system called Ptolemy II is used as a laboratory for exploring these concepts. This presentation uses Ptolemy II to illustrate and compare various popular models of computation, and to discuss how models of computation may be combined heterogeneously.