Evidence preserved in the petrology and chemical composition of erupted arc lavas provides the basis for understanding the processes that give rise to arc magmas. Work over the past 30 years has resulted in a preponderance of evidence to suggest arc parental magmas commonly contain up to 4–6 wt% H2O and some arc andesites contain up to 8–10 wt% H2O. However, considerable uncertainty remains about the physically and compositionally complex processes that lead to the generation of hydrous arc magmas with these observed water contents. In this talk, I will present new experimental evidence regarding the systematics of melting H2O-oversaturated and chlorite-bearing undepleted peridotite from 3 to 6 GPa. These experiments are then used to understand the temperatures and chemical reactions of mantle wedge melting that constitute the primary controls on (1) the location of arc volcanoes and (2) the width of the volcanic arc.