Most of the world’s great earthquakes are inter-plate underthrusting events in the subduction zones of convergent margins. As the December 2004 Sumatra earthquake and Indian Ocean tsunami demonstrated, subduction-zone earthquakes represent one of the greatest natural hazards on the planet. Large destructive earthquakes that occur on land release cumulatively far less seismic energy than subduction-zone earthquakes. Although plate tectonics provides the underlying kinematic explanation for subduction-zone earthquakes, only a narrow portion of the plate-contact zone actually generates them—the so-called seismogenic zone (Figure 1). Increased awareness of the destructive power of subduction-zone earthquakes has resulted in a rapidly growing research effort to learn about the mechanics and dynamics of faulting processes that integrate rock mechanics, seismology, geodesy, frictional physics, and fluid-fault interactions. To a first approximation, we understand that, because subduction-zone earthquakes are capable of rupturing large areas, they release the great majority of Earth’s seismic energy. We do not, however, understand the factors that occasionally lead some earthquakes to rupture extremely large areas, resulting in truly great (M > 9) subduction-zone events, while others rupture much smaller areas, producing events of M < 7.5. Furthermore, we do not know the relative roles of fault area, seismic coupling, seismic versus aseismic slip, asperities (areas of the two plates that are locked together), type and thickness of subducted sediments, and fluid flow.