The emergence of the MMIC as a cost effective, compact and enabling technology has increased the need for accurate CAD software. The performance of nonlinear MMICs must be evaluated during design using computer simulation, since they cannot be tuned after fabrication. Simulation relies upon accurate large-signal models for circuit components and this project involves the development of the GaAs MESFET large-signal model. In this work, the model is empirical and is derived entirely from characterizing S-parameter measurements over a range of bias levels and frequencies. Small-signal equivalent circuits are calculated from each set of S-parameter measurements and the nonlinear model is constructed from the complete set of equivalent circuits. Frequency dispersion in the conductances of the MESFET creates differences in the device characteristics at low and high frequencies. Extra nonlinear elements have been therefore added to the nonlinear model, to account for these effects. A series of MMIC circuits have been designed. Nonlinear measurements have been made and are compared with time domain simulations using the nonlinear model. Results indicate that this modelling approach is more accurate than one based on DC measurements, which does not account for the effects of frequency dispersion.