The interesting and complex biology of the interferon (IFN) system is briefly reviewed. Some encouraging results from the application of IFNs to therapy of human disease, as well as therapeutic problems are also pointed out. It is likely that increased knowledge of the functions of the IFN system will facilitate development of more efficient therapeutic strategies, involving e.g., mobilization of the patient's own ability to produce IFN, new combination therapies and circumvention of side effects such as development of antibodies neutralizing IFNs and autoimmunity.