Array antennas were used in early radar systems. Today's microwave phased arrays with electronic beam-steering allow surveillance and tracking with variable data rates in a multifunction radar. The paper reviews system options and discusses the engineering of active arrays, using as an example the Siemens Plessey Radar/ARE MESAR, which is now undergoing trials and which is the first modular active array radar in the UK. Future applications, the success of which depends on cost-effective production of active transmit–receive modules, improved beam control, and larger wideband arrays, are outlined.