This article presents an overview of (i) the structural features of the 3 types of mammalian interferon proteins (alpha, beta, and gamma) and of the gene families encoding them, (ii) the main biological effects and molecular events mediated in vitro and in vivo by these molecules and (iii) their present use in human and veterinary medicine. Recent results are detailed concerning the involvement of some interferon subspecies in physiological processes such as regulation of growth and differentiation of some cell lineages, interaction with other cytokines and possible hormonal effects of interferons.