Mantle plumes—which are usually, but not always, chemically distinct from the mid-ocean ridge basalt (MORB)—may be rooted in the core-mantle boundary and begin with large voluminous heads triggering massive eruptions or be headless and arise in the mid-mantle. Geochemistry provides convincing evidence that mantle plumes are 100-300°C hotter than normal upper mantle and that upwelling rates within the melting region are faster than beneath mid-ocean ridges. 186Os/188Os hints at the possibility of material from Earth's core in the Hawaiian plume, but this is not seen in other oceanic island basalt (OIB) and has not been confirmed by 182W/184W measurements. High 3He/4He in plumes does not require a primordial deep-mantle reservoir. The geochemical signature of mantle plumes originates primarily through melting in the upper mantle, probably through creation and subduction of oceanic lithosphere, but the details remain obscure. Plumes are lithologically heterogeneous, consisting of stringers of mafic material embedded in a more dominant peridotite.