A Marx generator is an attractive high-voltage pulse source for a High Power Microwave warhead since it can be built rather compact and can amplify the voltage by an order of magnitude or more.To design a compact and efficient Marx generator, it is essential to be able to simulate its behaviour. This report presents a circuit simulation model of a 20 stage Marx generator and a rapid charger along with selected simulation results. Simulations have been carried out of both charging and discharging of the Marx generator. Discharges into a resistive load and a vircator load have also been modelled. The Simulink model of a Marx generator discharge into a resistive load shows good agreement with experimental results. One of the most crucial components to model is the spark gap closing switch, since it determines the time it takes to fire the Marx generator and initiates the high frequency oscillations in the circuit. With a simple vircator model is load, the agreement with measured current and frequency is qualitatively correct. The model can be used for parameter studies of the Marx system and hence also for finding ways to improve its function and efficiency. The Marx model can easily be adapted to other Marx generator designs, with different numbers of stages, stage impedances, etc. The Simulation model can also be used as base for experimental and theoretical studies of various loads connected to a Marx generator.