This book discusses the immunotherapeutic potential of Interleukin 12 in the context of clinical oncology, as well as antitumor effects confirmed in preclinical studies and clinical trials in cancer immunotherapy. Due to its ability to activate both innate (NK cells) and adaptive (cytotoxic T lymphocytes) immunities, Interleukin 12 (IL-12) has been regarded as a promising candidate for tumor immunotherapy. However, despite the encouraging results in animal models, only very modest antitumor effects have been confirmed in early clinical trials. Recently, several clinical studies have been initiated in which IL-12 was applied as an adjuvant in cancer vaccines, in gene therapy including locoregional injections of IL-12 plasmid, and in the form of tumor-targeting immunocytokines (IL-12 fused to monoclonal antibodies).