The andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49--50% SiO2 to about 76% SiO2. The most primitive compositions, having contents of about 50% SiO2, are the basaltic andesites, which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust. The oceanic andesites cannot have assimilated material from the continental crusts, but their petrogenesis bears on the origin of the continential crust. The fractionation of andesitic magmas occurs at relatively low pressures, within the stability range of plagioclase. Basaltic andesites fractionate mainly orthopyroxene +plagioclase, while andesites fractionate clinopyroxene+orthopyroxene+plagioclase. The petrogenesis of the continental andesites is more complex, as the primary andesites contain sedimentary material from the continental crusts.