Belonging to the vast family of cytokines, interferons (IFN) have recently been widely investigated concerning their possible clinical applications, both in virology and oncology. In this field results have been quite mixed but definitely encouraging. The best achievements have been obtained in hematology, and particularly in the treatment of hairy cell leukemia and chronic myelogenous leukemia, but new perspectives have also opened in the therapy of solid tumors, especially in the local treatment of superficial bladder cancer and ovarian cancer, AIDS-related Kaposi's sarcoma and malignant melanoma. IFN have in certain cases showed an efficacy comparable to that of classic treatments but with lower toxicity, and in some tumors they have even improved the results obtained so far, especially in combined therapy. We have here gathered the most recent results concerning the use of IFN in the therapy of solid tumors in order to highlight the new therapeutic opportunities available to clinical oncology.