Interferon (IFN) was discovered by virologists interested in the phenomenon of viral interference and for many years was considered to be exclusively an antiviral substance. In time, it was accepted that the antiviral action was only one manifestation of the multiple effects of IFN on cells. IFN was shown to inhibit the development of tumors, to modulate immune function, and even to induce disease. Some aspects of these phenomena will be discussed. Despite this plethora of biological effects of type I IFN, the potency of its antiviral action, combined with its varied effects on both the cell-mediated and humoral immune response, render it a most effective and remarkable antiviral substance. Arguments will be presented to support the hypothesis that the major role of IFN is, after all, in antiviral defense. Finally, the use of type I IFN in the treatment of patients with viral and neoplastic diseases and even diseases of varied and unknown etiology is discussed.