This chapter provides an introductory review of processes involved in developing models for engineering applications. Current trends in the design of integrated, multidisciplinary and embedded systems, and the associated consequences for modelling and simulation, are emphasised. Ways of classifying models are presented and the experimental approach to model development through system identification and optimisation is introduced. Issues of model quality are emphasised, together with model reuse, the development of libraries of sub-models, the potential of generic descriptions and use of modelling within procurement. These topics are all considered in more detail in later chapters.