Basaltic volcanism provides a window into the Earth's mantle. Seeing through this window requires seeing through the processes of magma genesis and evolution that distort the view. Radiogenic isotope ratios, and to a lesser extent incompatible elements and stable isotope ratios, allow us to see through these distortions and infer the nature and evolution of various mantle magma sources. Geochemical studies of mid-ocean ridge basalts (MORB) reveal that the upper mantle was long ago depleted in incompatible elements through previous episodes of melt extraction, likely associated with crust formation. Oceanic island basalts (OIB) are the products of mantle plumes and hence provide us with a view of the deepest parts of the Earth's mantle. These plumes can be divided into several genera, EMI, EM II, and HIMU, most of which contain several species. Little or no mixing is observed between these genera, but all mix with a prevalent mantle component, PREMA, that contributes to many if not all plumes. Although the mantle comprising plumes also show evidence of previous melt extraction events, the more prominent chemical signature is of material anciently recycled from the Earth's surface through subduction of oceanic crust and sediment, subduction erosion, and foundering of lower continental crust. This is most unambiguously manifest in variations in stable isotope ratios that could only have been produced at or near the surface of the Earth. In addition to a recycled component, many plumes also contain a quite primitive component, manifested primarily in their noble gas isotope ratios. The primitive component seems most closely associated with PREMA.