Although tumor vaccines have been studied for decades, there is no vaccine approved as a clinical product. Nevertheless, recent advances in immunology and tumor biology justify a renewed interest. First, cancer cells express many antigens that can be recognized by the immune system, some with high tumor selectivity. Second, knowledge about immune regulation, including the importance of costimulatory signals, has been successfully applied to the studies of tumors. Third, mechanisms of how tumors can escape from immunological control have been identified, setting the stage to discover agents to decrease their impact. Rejection of established mouse tumors has been accomplished as a result of therapeutic tumor vaccination and there are encouraging findings from vaccine trials in humans.