  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 $\bar{\theta}$. To agree with experimental bounds, however,
this parameter must be very small $(\bar{\theta} \leq 10^{-9}$). After briefly
discussing some possible other 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.
