The strong CP problem and axions

I describe how the QCD vacuum structure, necessary to resolve the U(1)A problem, predicts the presence of a P, T, and CP violating term proportional to the vacuum angle (theta) over bar. To agree with experimental bounds, however, this parameter must be very small ((theta) over bar less than or similar to 10(-9)). After briefly discussing some other possible solutions. to this, so-called, strong CP problem, I concentrate on the chiral solution proposed by Peccei and Quinn which has associated with it a light pseudoscalar particle, the axion. I discuss in detail the properties and dynamics of axions, focusing particularly on invisible axion models where axions are very light, very weakly coupled, and very long-lived. Astrophysical and cosmological bounds on invisible axions are also briefly touched upon.