Microfabricated field emitter arrays have attracted interest for a variety of applications. The most prominent of these applications are flat panel displays, microwave amplifiers, x- ray tubes, electron beam probes, ionizers for vacuum pressure gauges, mass spectrometers, and electronic charge management on spacecraft. From a commercial point of view, the most exciting application has been flat panel displays, while high frequency applications are the most challenging with respect to cathode performance. Displays require attention to issues related to economic high-volume production, very low-voltage operation, and a very high level of uniformity over large areas with a low emission current loading. Microwave and other high frequency applications require small areas, with high tip packing density and the highest possible current loading per tip. Ionization and charge management applications require moderate emission performance, but present special problems with regard to stability and lifetime in relatively harsh environments. Designing an emitter array to meet the requirements of any of these applications involves dealing with lithography issues concerning emitter size and packing density; materials issues as they relate to fabrication processes; stability and lifetime issues with regard to hostile environments, and electronic properties such as dielectric constant, resistivity, and work function of the emitter tip; and the cost of large-scale production.