  The physical origin of the strong CP problem in QCD, rooted in the structures
of the vacuum of the standard model, is reviewed. The chiral solution to this
problem, with its accompanying axion, is explained and various characteristics
of axions are detailed. Although visible axion models are excluded
experimentally, models with very light and very weakly coupled axions are still
tenable. The astrophysical and cosmological implications of such axion models
are discussed, along with ongoing experimental attempts to detect such, so
called, invisible axions.
