Interferons (IFNs) display a pleiotropic effect on different cell types of both the innate and the adaptive immunities being able to affect the immune responses. The ability of IFNs, and in particular of type I IFN, to activate dendritic cells and to modulate the expression of major histocompatibility classes I and II molecules supports their potential role also in the development and maintenance of tolerance. When tolerance breakdown has occurred, immunocomplexes generated by the reaction of nuclear antigens and specific autoantibodies can further induce type I IFN production. Accordingly, high acid-labile type I IFN plasma levels, overexpression of IFNalpha-induced transcripts and the association with genes closely related to type I IFN response represent the rationale for the so-called IFN signature in systemic lupus erythematosus, a prototypical autoimmune disease. The role of IFNs in autoimmunity is further supported by their direct and deleterious impact on target tissues. The therapeutic use of IFNs is based on their antiviral and antiproliferative effect. Type I IFN administration, in particular, is associated with the appearance of autoimmunity, although less frequently than expected. Such an event takes place mostly in patients with previous autoimmune manifestations and can be characterized by the appearance of autoantibodies only or of a clinically overt disease. IFN therapy cessation is usually, but not always, required for controlling the autoimmune disorders.