就大部分实用的目的，月球都被认为是被真空环绕着。很严肃的与邻近存在的原子和分子比较 (相较于行星际介质)，相较于围绕在地球和太阳系内其它的行星的大气层，在科学目标的大气层，月球的大气层是可以忽略不计的－远低于地球海平面大气层密度的百兆分之一。

月球大气的一个来源是出气：来自地壳和地函中的氡和氦的放射性衰变所释放出的气体。另一个重要来源是撞击到月球表面的微陨石、太阳风和阳光，进行已知的溅射过程。溅射释放出的气体不是被月球的重力吸引重新回到风化层；就是因为辐射压而失落置太空中，或如果气体是被电离的，就会被太阳风的磁场扫掠进太空。

The atmosphere of the Moon is the tenuous layer of gases surrounding the Moon. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity compared to interplanetary medium, referred to as 'lunar atmosphere' for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar System—less than one hundred trillionth (10) of Earth's atmospheric density at sea level. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its atmosphere is lost to space.