Although ureilites have been considered bizarre meteorites, our analysis of their properties suggests that they formed by simple, well-known processes. We conclude that: (1) ureilites are the sole rock type in a major fraction of their parent body; (2) they are partial melt residues and not cumulates; (3) removal of 90% of the Al in the basalts would largely stop radioactive heating if 26Al was the heat source, ensuring that large volumes of ureilite-like material were generated; (4) they formed in a large chondritic body that had a heterogeneous O isotopic composition; (5) slow diffusion of O in solid and molten silicates ensured that partial melting did not homogenize the O isotopes of the residues; (6) the puzzling absence of the basaltic melts that accompanied ureilite formation probably results from ejection into space by explosive volcanism. Together, these conclusions provide a simple, plausible origin for ureilites that accounts for most of their properties: ureilites are residues from the partial melting of isotopically heterogeneous chondritic material. We suggest that other meteorite parent bodies and planetesimals may have experienced similar histories, and that ureilites provide important constraints on the accretionary and igneous histories of planets and asteroids. ¿ American Geophysical Union 1993