Quarkonia and their transitions

Valuable data on quarkonia (the bound states of a heavy quark $Q=c,b$ and the corresponding antiquark) have recently been provided by a variety of sources, mainly ${e}^{+}{e}^{\ensuremath{-}}$ collisions, but also hadronic interactions. This permits a thorough updating of the experimental and theoretical status of electromagnetic and strong transitions in quarkonia. The $Q\overline{Q}$ transitions to other $Q\overline{Q}$ states are discussed, with some reference to processes involving $Q\overline{Q}$ annihilation.