Tumors produce several factors, such as Prostaglandins (PGs), Interleukin (IL)-10, Vascular Endothelial Growth Factor (VEGF) and Transforming Growth Factor (TGF)-beta, which may directly or indirectly inhibit the immune response and may hamper immunotherapy. Furthermore, cells of innate or adaptive immunity, recruited by tumor-derived factors, may contribute in immunosuppression. Regulatory T (Treg) cells such as the "naturally occurring" CD4(+)/CD25(+) Treg and the IL-10-induced Tr1 cells are major players in this arena. Paradoxically Treg cells are stimulated by IL-2, which is used in tumor immunotherapy. Treg cells suppress T cell responses through soluble factors or by contact-dependent mechanisms, such as the Cytotoxic T Lymphocyte Antigen (CTLA)-4-mediated induction of Indoleamine 2,3-Dioxygenase (IDO) in dendritic cells (DC). IDO inhibits T cell responses by depleting Tryptophan and producing Kynurenine, which is toxic to lymphocytes. Macrophages, granulocytes or myeloid suppressor cells (MSC) suppress immunity by other enzymatic mechanisms, involving Arginase and Nitric Oxide Synthase (NOS). Subversion of tumor immunosuppression is required for successful immunotherapy. Attempts to block or eliminate Treg cells have been made by the use of chemotherapy, anti-CD25 or anti-CTLA-4 antibodies, IL-2-toxin chimeric proteins or Glucocorticoid-induced TNF-like Receptor (GITR) and CD134/OX-40 ligands. Tumor cells genetically modified to secrete IL-21 (an immune-stimulatory "IL-2-like" cytokine, which is not involved in immune regulation) cured experimental metastases in combination with anti-CD25 monoclonal antibodies (mAbs). Also strategies aimed at blocking enzyme-based immune-suppressive mechanisms are suitable, as suggested by experimental evidences in mouse tumor models.