Interferons are widely expressed cytokines that have potent antiviral, antiproliferative, and immunomodulatory effects. Type I interferons show complex biology; in some cases, they promote autoimmunity and inflammation, and in other cases, exhibit homeostatic functions by controlling inflammation and tissue destruction. This complexity is exemplified in the 2 major autoimmune diseases: systemic lupus erythematosus, in which type I interferons play an important role in the pathogenesis, and multiple sclerosis, in which interferon beta, a type I interferon, exhibits protective and therapeutic roles. This article reviews the basic clinical data on type I interferons in autoimmune diseases and type I interferons as potential targets for therapies in autoimmune diseases.