MicroElectroMechanical Systems (MEMS) are a burgeoning area of device development that has growing commercial applications in automotive, medical, and display technologies, with the promise of vigorous near-term expansion into areas such as information systems, chemical analysis, and consumer products. Microsystems present an arena for the emergence of new plasma processing techniques, especially in the demanding area of etching. Today's most prevalent IC-based fabrication method is multilayer polysilicon surface micromachining. Plasma etch processes for defining structures in surface micromachined films comprise critical fabrication steps and must evolve to enable advances in microsystem capability. Typical plasma etch processes in surface micromachining will be outlined in terms of requirements like etch profile, aspect ratio, selectivity, and etch rate for several illustrative MEMS devices. Future etch demands such as submicron spaces with aspect ratios greater than 10, polysilicon films more than 6 microns thick, and large mask open areas will be described. In fact, the needs for vertical sidewalls and simultaneous clearing of both small (1 micron) and large (>10 microns) spaces, without any micromasking, already challenge conventional plasma tools and processes. Lastly, extremely deep silicon etching for very high aspect ratio structures, or through-wafer vias for material or optical transmission, interconnect, or environmental exposure will be discussed as perhaps the most difficult task for plasma etching technology. Although the time-multiplexed "Bosch" process has been viewed as a major breakthrough for the deep silicon etch application, opportunities remain for refinements, or alternatives employing more conventional etch tools.@footnote 1@ @FootnoteText@ @footnote 1@Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.