The Earth’s Moon is the largest natural satellite in the inner Solar System. The Moon is also witness to more than 4.5 Ga of Solar System history and is the only planetary body other than the Earth for which we have collected samples from known locations. Moreover, the lunar surface preserves a record of the cratering rate and the evolution of solar and galactic cosmic radiations throughout the history of the Solar System. Understanding the Moon is essential to understanding both the Earth and our Solar System. Consequently, the Moon was the prime target in Solar System exploration programs, before the pursuit of more distant targets such as Mars and beyond.

Our knowledge about the Moon is based on telescopic observations from the Earth, observations by spacecraft from the lunar orbit, measurements on the lunar surface by manned and unmanned lander missions and the analyses of lunar samples in terrestrial laboratories. The knowledge gained from the Apollo and Luna programs of the 1960s and subsequent lunar missions, carried out over the last four decades, continues to demonstrate the value of the Moon in the understanding of our Solar System and the fundamental processes that drive planetary formation and evolution.

Because of its restricted geological activity and relatively simple composition compared with the Earth, the Moon provides insights into elementary planetary processes. In comparison to the Earth, the Moon is depleted in both volatile elements, and iron and other siderophile elements. Recently, however, the presence of H2O and OH has been confirmed on the lunar surface as well as in lunar samples. While it has long been suspected that water-ice might be preserved in cold traps at the lunar poles, recent results indicate the presence of OH and H2O outside of these regions. This new discovery makes the Moon an extremely interesting target once again, both scientifically and as a potential resource. Although new data have helped to address some of our questions about the Earth-Moon system, major new questions have emerged and many existing ones remain unanswered.