After decades of speculation and fruitless searches by observers, the lunar atmosphere was first observed by Apollo surface and orbital instruments beginning in 1971. With the end of Apollo missions in 1972 and the termination of funding for Apollo lunar ground station observations in 1977 the field withered for many years, but it has recently enjoyed a renaissance. This renewal was initiated by the discovery of lunar atmospheric sodium and potassium by ground-based observers and was furthered by the in situ detection of metal ions derived from the Moon in interplanetary space, the possible discoveries of H2O ice at the poles of the Moon and Mercury, and the detection of tenuous atmospheres around other remote sites in the solar system, including Mercury and several Galilean satellites. In this review I attempt to summarize the present state of knowledge about the lunar atmosphere, describe the important physical processes taking place within it, and compare the lunar atmosphere with other tenuous atmospheres in the solar system. ¿ 1999 American Geophysical Union