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Models of computation (MoC) are reviewed and organised with respect to the time abstraction they use. Continuous time, discrete time, synchronous and untimed MoCs are distinguished. System level models serve a variety of objectives with partially contradicting requirements. Consequently, it is argued that different MoCs are necessary for the various tasks and phases in the design of an embedded system. Moreover, different MoCs have to be integrated to provide a coherent system modelling and analysis environment. The relation between some popular languages and the reviewed MoCs is discussed to find that a given MoC is offered by many languages and a single language can support multiple MoCs. It is contended that it is of importance for the quality of tools and overall design productivity, which abstraction levels and which primitive operators are provided in a language. However, it is observed that there are various flexible ways to do this, e.g. by way of heterogeneous frameworks, coordination languages and embedding of different MoCs in the same language.