There is possibly no more important chemical component in magmas than H2O. Water dramatically lowers the melting temperature of the mantle, has a major effect on the rheological properties of both magmas and crystals, transports chemical components between the hydrosphere, mantle, and crust, and causes the vesiculation which fuels explosive eruptions. The last fifteen years have seen an explosion of data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions trapped in volcanic phenocrysts, aliquots of magma that have escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations in the least degassed melt inclusions from each volcano. Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6 wt% H2O. The average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with a global average of 3.9 +/- 0.4 wt% H2O. The narrow range and common average value for H2O are in stark contrast to the concentrations of most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of arc melt inclusions. Either the crust imposes a common water content to pre-eruptive stored magma, or the mantle delivers a uniform product. The characteristic mean and range of H2O contents of arc magmas has implications for both the volatile fuel for explosive eruptions and the mass balance of H2O recycled through subduction zones.