In a cooling solar nebula, five of the six platinum-group elements (PGE) condense as refractory-metal alloys at temperatures above the condensation of Fe-Ni metal. Non-refractory Pd condenses in solid solution with Fe-Ni. Such refractory alloys are preserved in some meteorites, although they are often highly altered. The high resistance of PGE to oxidation leads to efficient extraction with metallic Fe-Ni during metal segregation and core formation. Experimentally determined PGE metal-silicate partition coefficients predict lower contents of PGE in planetary silicates than are found, supporting a late addition of PGE components. PGE are particularly useful as tracers of impacting extraplanetary materials in the strongly PGE-depleted crusts of the Earth and other planets.