Subduction zones are where sediments, oceanic crust, and mantle lithosphere return to and reequilibrate with Earth's mantle. Subduction zones are interior expressions of Earth's 55,000 km of convergent plate margins and are the geodynamic system that builds island arcs. Excess density of the mantle lithosphere in subduction zones provides most of the power needed to move the plates while inducing convection in the overriding mantle wedge. Asthenospheric mantle sucked toward the trench by the sinking slab interacts with water and incompatible elements rising from the sinking plate, and this interaction causes the mantle to melt. These melts rise vertically through downwelling mantle to erupt at arc volcanoes. Subduction zones are thus interior Earth systems of unparalleled scale and complexity. Subduction zone igneous activity formed most ore deposits and continental crust, and earthquakes caused by the downgoing plate present a growing hazard to society. This overview summarizes our present understanding of subduction zones, using perspectives of the incoming plate, downgoing plate, mantle wedge, and arc-trench complex. Understanding the operation of subduction zones stands as one of the great challenges facing the Earth sciences in the 21st century and will require the efforts of global interdisciplinary teams.