Variations in the thickness of the lithosphere are likely to influence the trace element compositions of basaits by controlling the distribution of melt and the minerals in the melting assemblage in the asthenosphere. Examples of systematic trace element variations that appear to be related to the thickness of the lithosphere are found in both oceanic and continental basalts. In the oceans, lithospheric thickness is closely related to age, and its control is manifested by a relation between trace element composition and age of basement ocean crust in Atlantic ocean-island basalts. Beneath continents, lithospheric thickness is less age sensitive, but it varies widely in response to extensional tectonics. Contemporary hot spots, which have been implicated in the generation of ancient flood basalts, show variations in the rare earth element content of their associated basalts which apparently reflect changes in lithospheric thickness between the time of flood-basalt eruption and the present.