  The QCD axion was originally predicted as a dynamical solution to the strong
CP problem. Axion like particles (ALPs) are also a generic prediction of many
high energy physics models including string theory. Theoretical models for
axions are reviewed, giving a generic multi-axion action with couplings to the
standard model. The couplings and masses of these axions can span many orders
of magnitude, and cosmology leads us to consider several distinct populations
of axions behaving as coherent condensates, or relativistic particles. Light,
stable axions are a mainstay dark matter candidate. Axion cosmology and
calculation of the relic density are reviewed. A very brief survey is given of
the phenomenology of axions arising from their direct couplings to the standard
model, and their distinctive gravitational interactions.
