This overview describes the development of interferon therapy in four different types of viral diseases. (1) Upper respiratory infections: despite extensive efforts interferons have not found a place in the treatment of these very common diseases. (2) Herpes keratitis: alpha interferons are highly active in combination therapy, but have gained only very limited clinical use. (3) Papillomavirus infections: alpha interferons have been approved for the treatment of papillomavirus infections of skin and mucous membranes and are in fairly wide clinical use. (4) Chronic hepatitis B, C and D: alpha interferons have become the treatment of choice and are used very extensively worldwide. The four examples illustrate both the clinical potentials and the limitations of alpha interferons and give some guidelines for future work. The overall conclusion is that chronic viral diseases lend themselves to interferon therapy more readily than acute viral infections. The general trend is toward the use of interferons in combination therapy.