The results from in vitro immunological experiments, murine tumor models and patients with cancer clearly demonstrate that tumors have multiple mechanisms to evade the immune response. During the early stages of tumor development malignant cells can be poor stimulators, present poor targets or become resistant to the innate immune response, while at later stages, progressively growing tumors impair the adaptive immune response by blocking the maturation and function of antigen presenting cells and causing alterations in T cell signal transduction and function. Preliminary results also suggest a correlation between some of these changes and an increased metastatic potential of the tumor cells, a diminished response to immunotherapy, and poor prognosis. Carefully coordinated basic research studies and clinical immunotherapy trials will be required to fully determine the impact on the outcome of the disease and the response to treatment. However, understanding the mechanisms used by tumor cells to evade the immune system could result in new therapeutic approaches for preventing and/or reversing these immune alterations and have the potential of improving the current results of immunotherapy trials.