Some aspects of the numerical modelling of diamond deposition plasmas generated using microwave cavity systems are discussed. The paper mainly focuses on those models that allow (i) designing microwave cavities in order to optimize the power deposition in the discharge and (ii) estimating the detailed plasma composition in the vicinity of the substrate surface. The development of hydrogen plasma models that may be used for the self-consistent simulation of microwave cavity discharge is first discussed. The use of these models for determining the plasma configuration, composition and temperature is illustrated. Examples showing how to use these models in order to optimize the cavity structure and to obtain stable process operations are also given. A transport model for the highly reactive H2/CH4 moderate pressure discharges is then presented. This model makes possible the determination of the time variation of plasma composition and temperature on a one-dimensional domain located on the plasma axis. The use of this model to analyse the transport phenomena and the chemical process in diamond deposition plasmas is illustrated. The model is also utilized to analyse pulsed mode discharges and the benefit they can bring as far as diamond growth rate and quality enhancement are concerned. We, in particular, show how the model can be employed to optimize the pulse waveform in order to improve the deposition process. Illustrations on how the model can give estimates of the species density at the growing substrate surface over a wide domain of deposition conditions are also given. This brings us to discuss the implication of the model prediction in terms of diamond growth rate and quality.