The upper mantle is the source of almost all magmas. It contains major transitions in rheological and thermal behaviour that control the character of plate tectonics and the style of mantle dynamics. Essential parameters in any model to describe these phenomena are the mantle's compositional and thermal structure. Most samples of the mantle come from the lithosphere. Although the composition of the underlying asthenospheric mantle can be estimated, this is made difficult by the fact that this part of the mantle partially melts and differentiates before samples ever reach the surface. The composition and conditions in the mantle at depths significantly below the lithosphere must be interpreted from geophysical observations combined with experimental data on mineral and rock properties. Fortunately, the transition zone, which extends from approximately 410 to 660 km, has a number of characteristic globally observed seismic properties that should ultimately place essential constraints on the compositional and thermal state of the mantle.