The interaction of light with matter involves the electronic structure of the matter. In particular, the interaction is affected by the magnetic state of the medium, and this gives rise to the magnetooptical effects. The Faraday, Kerr, and Voigt effects are described, and their derivation from Maxwell's equations and the constitutive relations are given. The microscopic origins of the effects are discussed heuristically. A few representative applications of the magnetooptic effects to magnetic recording and to the control of light are explained.