  Lunar laser ranging has provided many of the best tests of gravitation since
the first Apollo astronauts landed on the Moon. The march to higher precision
continues to this day, now entering the millimeter regime, and promising
continued improvement in scientific results. This review introduces key aspects
of the technique, details the motivations, observables, and results for a
variety of science objectives, summarizes the current state of the art,
highlights new developments in the field, describes the modeling challenges,
and looks to the future of the enterprise.
