Hydrous arc magmas are produced when water-bearing materials from subducted oceanic plates are transported to the mantle beneath volcanic arcs, though the mechanism of mass transport remains debated. The geochemical characteristics of the slab component have important implications for the thermal structures of down-going plates and the fluxes of elements into the deep mantle. If slab temperatures are low, then elemental fluxes from the slab will be carried in a dilute fluid. If temperatures are high, the slab may melt instead. While a long-standing paradigm for arc volcanism has been that sediments melt and ocean crust dehydrates, a growing body of evidence from arc geochemistry and experimental petrology suggests both sediment and ocean crust melt. The low solubility of many elements in aqueous fluids prevents them from making a substantial contribution to arc mass-balance. Constraints from Sr concentrations and 87 Sr/ 86 Sr ratios require a large flux of Sr from the ocean crust, which is only possible if the crust melts. H 2 O/Sr ratios of arc volcanics are also inconsistent with slab fluids. These conclusions are supported by thermo-mechanical models indicating that slab temperatures exceed the hydrous solidus for both ocean crust and sediments. Examination of experimental data shows a likely strong effect of oxygen fugacity on residual phases during slab melting. Arc data are best explained if the ocean crust melts beneath all arcs under oxidizing conditions somewhere between FMQ and NNO+2. Experimental constraints on sediments also require melting and that sediment melt compositions depend on bulk composition as well as temperature. If these experiments serve as analogs to sediment melting beneath arcs, then sediment bulk compositions are a necessary input for any rare earth element-based slab thermometer. We present compositions for ocean crust partial melts and partition coefficients for sediment melting based on existing experiments, physical models, and arc data, that can be used in geochemical models of...