  A short review of the optical and far-infrared measurements on MgB$_{2}$ is
given. Multiband and multigap effects are analyzed by comparing optical
properties with other experiments and {\em ab initio} calculations. The covered
topics are: the plasma frequency, electron-phonon interaction, impurity
scattering, the effects of C and Al substitution, interband transitions and the
far-infrared signatures of the superconducting gaps.
