Bound states of heavy quarks in QCD

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.