The compositions of mafic volcanic rocks of the Kenya Rift Valley indicate either generation, or final equilibration, of magmas within the lithosphere. The dominant lithologies in the mantle sources are spinel- and garnet-lherzolites and melt generation spanned the spinel-garnet transition. Modally metasomatised sources have been less important, at least in Quaternary times. The sources are depleted relative to the sources of oceanic island basalts and heterogeneous as regards minor and trace element distribution. Variable degrees of partial melting (always < 3%) and retention of minor phases diversify magma compositions. Primary magmas are rare in the rift zone. All major sequences show evidence of extensive polybaric fractionation within the upper mantle and lower crust. Inter-crustal fractionation has commonly been accompanied by assimilation and the development of silica-(over)saturated liquids. Fractionation at high crustal levels has generated a wide range of mugearitic, trachytic and phonolitic magmas. Crustal anatexis has locally resulted in the formation of peralkaline rhyolites. Generation of the enormous volumes of rift volcanics must also have generated huge volumes of mafic and ultramafic cumulates, especially at or near the Moho. It is possible that, in the most magmatically active areas, more than 20% of the thickness of the crust is due to magmatic underplating and crustal intrusion.