There is abundant clinical and laboratory evidence that host defenses are a significant factor in the progression and the control of cancer. The host immune system plays the major role, primarily through cell-mediated responses to the neoplastic threat. Great progress has been made in evaluating immune competence, although the specific defects have not been identified in all patients. The history of immunotherapy includes numerous trials, with few successes and many failures. Advances in knowledge and technology, however, indicate that efforts to manipulate and strengthen host defenses have great potential in the fight against cancer.