A review of microwave devices for power generation and low-noise amplification in active array modules is given. Various configurations of active array module are discussed and particularly full attention is given to the use of microwave phase-locked loops to achieve the required gain and accurate phase control. The noise performance and response to pulse-compression waveforms for such loops is also considered. It is suggested that considerable benefits may be obtained by performing phase shifting and signal processing at i.f. or baseband and that this leads to the possibility of optical-fibre interfaces with modules, much simplifying the interconnection problem. The realisation of a phase-locked loop module with phase shift applied at i.f. and constructed in thick-film microstrip form for use at X band is described.