Lunar compositional constraints are derived on the basis of geophysical data (in particular, the lunar seismic models as revised by Nakamura) and petrological arguments. Only in the case of extreme assumptions can critical aspects of bulk lunar composition be demonstrated to be equivalent to the present-day terrestrial mantle; specifically, the Moon has an Mg number that is too low and an alumina abundance that is too high. Over a broad range of crustal densities the presence of a metallic core at least 150 km in radius is necessary to reconcile the upper mantle lunar seismic model with mass and moment of inertia constraints. A significant seismic discontinuity at 500 km depth may mark the lowest extent of early lunar differentiation, possibly representing a transition between highly fractionated upper mantle and less fractionated, perhaps even primordial, middle/lower mantle. ¿ American Geophysical Union 1988