  Lunar laser ranging (LLR) is used to conduct high-precision measurements of
ranges between an observatory on Earth and a laser retro-reflector on the lunar
surface. Over the years, LLR has benefited from a number of improvements both
in observing technology and data modeling, which led to the current accuracy of
post-fit residuals of ~2 cm. Today LLR is a primary technique to study the
dynamics of the Earth-Moon system and is especially important for gravitational
physics, geodesy and studies of the lunar interior. LLR is used to perform
high-accuracy tests of the equivalence principle, to search for a
time-variation in the gravitational constant, and to test predictions of
various alternative theories of gravity. On the geodesy front, LLR contributes
to the determination of Earth orientation parameters, such as nutation,
precession (including relativistic precession), polar motion, and UT1, i.e.
especially to the long-term variation of these effects. LLR contributes to the
realization of both the terrestrial and selenocentric reference frames. The
realization of a dynamically defined inertial reference frame, in contrast to
the kinematically realized frame of VLBI, offers new possibilities for mutual
cross-checking and confirmation. Finally, LLR also investigates the processes
related to the Moon's interior dynamics. Here, we review the LLR technique
focusing on its impact on relativity and give an outlook to further
applications, e.g. in geodesy. We present results of our dedicated studies to
investigate the sensitivity of LLR data with respect to the relativistic
quantities. We discuss the current observational situation and the level of LLR
modeling implemented to date. We also address improvements needed to fully
utilize the scientific potential of LLR.
