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The use of gallium arsenide as a material for monolithic microwave circuits where active devices, such as f.e.t.s, diodes etc., are integrated onto the same piece of material as passive components is now receiving considerable attention in Europe and the USA. The paper concentrates on the specific role of monolithic circuits in phased-array-radar applications with descriptions of the use of f.e.t.s in switches, phase shifters, attenuators, receivers and transmitters. A summary of GaAs f.e.t. device performance is included allowing some insight into the noise figures, output powers and efficiencies obtained from low-noise and power amplifiers, respectively, at frequencies in S- and X-band. Some examples of GaAs monolithic circuit designs are given and methods of using active as against passive matching to achieve higher packing densities are described. The yield and cost of monolithic techniques is reviewed in the light of present and predicted circuit design and technology improvements. The impact of such costs on the realisation of phased-array systems with large numbers of elements is reviewed.