The most common eruptions observed by humans, and by far the most dangerous to human populations, are those from volcanoes above the world's subduction zones (Simkin and Siebert, 2000). Population growth and development of technology are also concentrated in areas such as the Pacific Rim, where subduction-zone volcanism is prevalent. Many new and proposed nuclear facilities are therefore located in regions of active subduction (Connor et al., Chapter 3, this volume). Because nuclear facilities require low-risk sites, and because some nuclear facilities, such as high-level radioactive waste repositories, require very long performance periods, it is necessary to understand the nature of volcanism in subduction zones from a regional, plate tectonic perspective. This perspective will allow us to develop more robust hazard models for future volcanic activity on a variety of timescales, and to better assess assumptions made by these volcanic hazard models. The goal of this chapter is to provide state-of-the-art information about the geological processes operating on a regional scale in subduction zones. Subduction zones are locations where oceanic plates subduct into the mantle; they are characterized geomorphologically by deep ocean trenches and volcanic arcs or continental margins, seismically by landward-dipping deep seismic zones and magmatically by arcuate belts of volcanoes. Subduction and arc magmatism are fundamental processes in the evolution of the Earth, because they play crucial roles in the present-day differentiation of Earth's materials and are believed to be major sites of continental crust generation that have operated throughout geologic time (e.g. Taylor, 1967; Arculus, 1981; Gill, 1981; Eiler, 2003; Rudnick and Gao, 2003).