In the period since the mid-70's there have been rapid technology advances for the millimeter-wave field, and these have led to investigations of phenomenology (target and background signatures, and propagation effects in smoke, dust, or inclement weather). Extensive use of this information is now being made in applications such as communications, radar, guidance, imaging, electronic warfare and radio astronomy.l Sources are available which can in some cases provide an order of magnitude more power, significantly longer life-times, or complex waveforms such as pulse compression or coherent MTI for radar applications, and in addition usable power has become available at higher frequencies. Furthermore, over a period of years the sensitivities of receivers have improved dramatically, with noise temperatures for uncooled mixers decreasing by nearly an order of magnitude, and cooled mixers reaching noise temperatures below 75°K near 100 GHz. The use of such structures as beam-lead detectors has improved the mechanical performance and reliability, too. Progress has been made in component development, also, with particular activity in quasioptical components, specialized transmission lines (such as suspended substrate micro-strip, fin-line, dielectric image line, and insular guide), and integrated and monolithic circuits. The following discussion reviews much of the recent technical progress in the field, and then gives a summary of several major application areas, such as missile guidance.