Interleukin-12 (IL-12) is known for several years to have an essential role in inflammatory responses and innate resistance to infection and cancer. This has been largely attributed to its ability to initiate the differentiation of T-helper-1 (Th1) cells producing interferon-gamma. Recently, two new cytokines, IL-23 and IL-27, with homology to IL-12 were discovered and assigned to the IL-12 family of cytokines. Growing evidence supports a role for IL-23 as key mediator of autoimmune disease regulating the new Th17 subset of CD4+ T cells. IL-27 can have pro- and anti-inflammatory effects, which increase Th1 differentiation, suppress Th2 proliferation, or stimulate cytotoxic T cell activity. Several strategies have been pursued to apply the immunological effects of IL-12 family members to the treatment of human disease. Whereas the inhibition of IL-12 and IL-23 signal transduction has shown promising results for the treatment of autoimmune disease, the administration of IL-12 during infection and cancer can increase the host immune reaction. The increasing knowledge about the new IL-12 family members, IL-23 and IL-27, has revealed new therapeutic options for the use of these cytokines. In this review, we discuss therapeutic strategies using IL-12 family members in infection, autoimmunity, and cancer with special focus on gene administration.