Discovered almost four decades ago, interferons were being investigated as potential anticancer agents from early sixties, because of their antiviral antiproliferative and immunomodulating properties and their relatively modest toxicity. Unlike very good effects on some lymphoproliferative disorders where complete durable remissions were observed, results of their application in solid tumors are not so unambiguous. Among large number of tumors investigated so far, the best, reproducible, although modest, activity was found against melanoma, renal cell carcinoma and Kaposi's sarcoma. There are still many doubts concerning their optimal doses, sequencing and schedules, most probably in combination with other biological response modifiers and/or cytotoxic agents, which have to be clarified in further clinical trials, together with investigation of more reliable biological indicators of response to interferons in vivo.