The composition of the bulk silicate Earth (BSE) is the product of planetary accretion, core differentiation and Moon formation. By establishing the composition of the BSE, one can determine the composition of the bulk Earth and by subtraction, calculate the core’s composition. The BSE represents the bulk Earth minus the core, which in today’s terms equals the modern mantle, the continental crust, and the hydrosphere-atmosphere systems. The modern mantle can be framed in terms of two compositionally distinct components, an enriched and a depleted mantle, with the latter as the MORB (mid-ocean ridge basalt) source and the former as the OIB (ocean island basalt) source.