This article explains the rationale for development of interferons as therapeutic agents, and describes commercial products available today. It also provides a summary of studies that have been performed with interferons for use as exogenous biological response modifiers in viral infections. Overall, the best data exist for treatment of viral hepatitis B and C, for which interferons are a cornerstone of therapy. Although infections with human papillomavirus and common cold viruses sometimes respond favorably to interferons, their outcomes are far from ideal. Finally, the role of interferons as vaccine adjuvants is still being explored but could be promising.