Dissolved volatiles can have tremendous effects on the physical and chemical properties of silicate melts. The most abundant volatile in terrestrial magmas is H2O. A few weight percent of added H2O can reduce melting temperatures of rocks by several hundred degrees and enhance the fluidity of magmas by orders of magnitude. Carbon dioxide and sulfur, although less abundant in natural magmas than H2O, often control the initial stage of magma degassing. The strong effect of volatiles on melt properties is related to the chemical bonding of the volatiles in the melt, which depends in particular on melt composition, temperature and oxygen fugacity. The oxygen fugacity, although very low at magmatic conditions, nevertheless has a large influence on the magma, determining the abundance and composition of minerals, fluid-melt partitioning and the physical properties of the melt.