Numerical modelling of plasma systems is proving an increasingly valuable tool in the understanding of fundamental processes in reactors for surface engineering applications. The present paper discusses some of the fundamental physical and mathematical models used to describe plasma behaviour and their range of validity and applicability to plasma reactors. A number of specific examples which illustrates the way in which these methods may be implemented to model plasmas in dc and rf diode reactors and plasma ‘ball’ microwave reactors are described, with comparison between numerical results, diagnostic measurements and experimental observations from the published literature.