Recent data on the concentrations of the refractory siderophile (metal-loving) elements molybdenum and rhenium confirm that most siderophile elements in lunar crustal rocks and mare basalts have significantly lower concentrations than in the earth's mantle and much lower concentrations compared to chondrite meteorites. The observed depletion of siderophile elements in lunar samples suggests the existence of a metal core. The amount of metal in the core depends on the conditions under which metal segregation occurred. If the moon accreted directly from the solar nebula, with chrondritic abundances of siderophile elements, then 2% to 5.5% metal must have segregated within the moon to form a core. Geophysical techniques constrain the radius of the lunar core to 500 km (5.5 wt%) or less. If the moon formed after the earth, from material out of the earth's mantle, segregation of 0.1% to 1% metal within the moon is required. Less metal is needed for a terrestrial origin because the earth's mantle was already partially depleted in siderophile elements due to formation of the earth's core. The siderophile elements therefore provide strong geochemical evidence for a lunar metal core, but do not rule out either formation of the moon from the earth's mantle or an independent origin by accretion from the solar nebula.