  Bound states of heavy $\bar{q}q$ quarks are reviewed within the context of
QCD, paying attention to what can be derived from the theory with a reasonable
degree of rigour. This is compared with the results of semiclassical arguments.
Among new results, we report a very precise $O(\alpha_s^4)$ evaluation of $b,
c$ quark masses from quarkonium spectrum with a potential to two loops.
