The moon is an attractive location for astronomical observatories because of the absence of a substantial lunar atmosphere and the stability of the lunar surface. The present lunar atmosphere is sufficiently transparent that there is no significant image distortion due to absorption or refraction. This thin atmosphere results from a combination of small sources and prompt losses. The solar wind is the major source, with a total mass input into the lunar atmosphere of less than 50 gm/sec. The principal atmospheric loss mechanism for most gases is photoionization within a period of weeks to months, followed by immediate loss to the solar wind. Lunar base activities will add gases to the lunar atmosphere by rocket exhaust, processing of lunar materials, venting of pressurized volumes, and astronaut life support systems. Although these sources will substantially exceed natural sources, for modest lunar base activity the effects are expected to be localized and transient and not a significant impediment to lunar based astronomy. The absence of a substantial lunar magnetic field or atmosphere allows solar wind, magnetospheric plasma, and cosmic radiation to penetrate directly to the lunar surface. This ionizing radiation environment creates difficulties for workers and some sensitive instruments, but for certain science investigations (such as neutrino detection) the background environment on the moon is lower than on the earth.