Knowledge of the lunar regolith is essential to success in lunar missions whether crewed or robotic. The regolith is the loose material overlying more intact strata on the Moon. It varies in thickness from several meters on the maria or lunar seas to many meters on the highlands of the Moon. The regolith is the material humans walked and drove on from 1969 to 1972. In the future, people will use it for radiation protection and as a resource for recovery of oxygen, silicon, iron, aluminum, and titanium. Implanted in the regolith by the solar wind are recoverable amounts of volatiles such as hydrogen and helium. Increasing our knowledge of the mechanical properties of the regolith will enable constructors of the 21st Century to build habitats, do mining, establish manufacturing, and erect telescopes on the Moon. We already know much of the regolith from robotic and astronaut missions to the Moon. There is much more to be learned.