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Over approximately the past decade, a subfield of plasma science has arisen that is redefining frontiers in the physics of low temperature plasma and its applications. Concerned with the confinement of weakly ionized, nonequilibrium plasma to cavities having mesoscopic dimensions, the emerging area of microcavity plasmas has advanced rapidly in surpassing several milestones, primarily with respect to electron density and cavity geometries, and is establishing new avenues of research. To date, peak electron densities above 1017 cm-3, cavity dimensions as small as 3 μm , microchannel aspect ratios (length: width) of 103:1 , plasma packets propagating at velocities up to 20 km s-1 , and coupling between e--h+ and e--ion plasmas have all been observed, but every indication is that these results are only a foretaste of the future. This review describes several recent device geometries and provides a synopsis of the physics. Promising applications of this technology in chemical processing, lighting, water disinfection, and medicine are also discussed briefly.