The Earth's natural satellite (Table 1). United States and Soviet spacecraft have obtained lunar data and samples, and Americans have orbited, landed, and roved upon the Moon (Fig. 1). After a long hiatus, lunar exploration by spacecraft has resumed in the first decade of the twenty-first century and there is even the prospect that humans may revisit and eventually reside upon the Moon. Though the first wave of human exploration has passed, it left a store of information whose meanings are still being deciphered. Many of the Moon's properties are now well understood, but its origin and relations to other planets remain obscure. Theories of its origin include independent condensation and then capture by the Earth; formation in the same cloud of preplanetary matter with the Earth; fission from the Earth; and formation after the impact of a Mars-sized body on the proto-Earth. Because many of the Moon's geologic processes stopped long ago, its surface preserves a record of very ancient events. However, because the Moon's rocks and soils were reworked by geochemical and impact processes, their origins are partly obscured, so that working out the Moon's early history remains a fascinating puzzle.