Documenting the mass flux through convergent plate margins is important to the understanding of petrogenesis in arc settings and to the origin of the continental crust, since subduction zones are the only major routes by which material extracted from the mantle can be returned to great depths within the Earth. Despite their significance, there has been a tendency to view subduction zones as areas of net crustal growth. Convergent plate margins are divided into those showing long-term landward retreat of the trench and those dominated by accretion of sediments from the subducting plate. Tectonic erosion is favored in regions where convergence rates exceed 6 ¿ 0.1 cm yr-1 and where the sedimentary cover is 1 km. Large volumes of continental crust are subducted at both erosive and accretionary margins. Average magmatic productivity of arcs must exceed 90 km3 m.y.-1 if the volume of the continental crust is to be maintained. Convergence rate rather than height of the melting column under the arc appears to be the primary control on long-term melt production. Oceanic arcs will not be stable if crustal thicknesses exceed 36 km or trench retreat rates are >6 km m.y.-1. Continental arcs undergoing erosion are major sinks of continental crust. This loss requires that oceanic arcs be accreted to the continental margins if the net volume of crust is to be maintained.