A recent, important development in low-pressure and low-temperature plasma processing is the microwave electron cyclotron resonance (ECR) discharge. Its lack of electrodes and its ability to create high densities of charged and excited species at low pressures (≲10−4 Torr) make it an attractive processing discharge in etching and thin-film deposition applications. This article reviews the basic physics of ECR discharges and reviews the associated microwave system and applicator technologies. Waveguide and cavity ECR applicators are compared and are described in detail. Several ECR plasma processing reactors are also described. Methods of processing large surfaces are outlined, and typical experimentally measured ECR discharge characteristics are presented.