  Baryon Acoustic Oscillations (BAO) are frozen relics left over from the
pre-decoupling universe. They are the standard rulers of choice for 21st
century cosmology, providing distance estimates that are, for the first time,
firmly rooted in well-understood, linear physics. This review synthesises
current understanding regarding all aspects of BAO cosmology, from the
theoretical and statistical to the observational, and includes a map of the
future landscape of BAO surveys, both spectroscopic and photometric.
