The impact history of the Moon has significant implications that go far beyond simply excavating the surface of a dry and lifeless world. The age distribution of lunar impact breccias inspired the idea of a catastrophic influx of asteroids and comets about 4 billion years ago and motivated new models of planetary dynamics. A late bombardment may have regulated environmental conditions on the early Earth and Mars and influenced the course of biologic evolution. The cataclysm hypothesis is controversial, however, and far from proven. Lunar explorers face the difficult task of establishing absolute ages of ancient impact basins and the sources for the impactors.