During the past ten years electron cyclotron resonance (ECR) plasma-processing technology has matured into a diverse assortment of ECR plasma reactor and plasma source design concepts and has been extensively applied to numerous low-pressure plasma processing applications. This paper reviews the substantial progress made in the design and application of ECR plasma technology in recent years. Five representative ECR reactor/source designs from large-area 450-cm2 discharges to compact plasma sources inserted into molecular-beam epitaxy (MBE) machines are described in detail. The performance of these ECR devices is evaluated by computing performance figures of merit from the available experimental data. These calculations are then compared with the behavior as predicted from a global model of the discharge. This comparison suggests that global plasma models can be employed as an approximate method for ECR reactor design. More extensive diagnostics and numerical models that investigate the spatial variation of ion density and ion energy distributions are also presented. Several illustrative ECR plasma-processing applications are discussed. These include submicron etching of silicon, etching of III-V and II-VI electronic and photonic devices, and the epitaxial growth of GaN. The variety and the sophistication of these applications demonstrate that low-pressure high-density ECR plasma processing technology has evolved into a very useful, versatile group of plasma-processing machines. © 1997 IEEE.