  Ground-based optical astronomy necessarily involves sensing the light of
astronomical objects along with the contributions of many natural sources
ranging from the Earth's atmosphere to cosmological light. In addition,
astronomers have long contended with artificial light pollution that further
adds to the 'background' against which astronomical objects are seen.
Understanding the brightness of the night sky is therefore fundamental to
astronomy. The last comprehensive review of this subject was nearly a
half-century ago, and we have learned much about both the natural and
artificial night sky since. This Review considers which influences determine
the total optical brightness of the night sky; the means by which that
brightness is measured; and how night sky quality is assessed and monitored in
the long term.
