The early and recent investigations in the field of retinoids and cancer are reviewed. The retinoids, including natural vitamin A compounds and their synthetic analogs, present a new class of substances exerting a prophylactic and a therapeutic effect both in certain experimental tumor models and in certain clinical conditions of preneoplastic and neoplastic lesions. Because of a particular physiological mechanism of action, the retinoids offer a new approach to the cancer problem, which is different from those of surgery, X-ray therapy, conventional chemotherapy, and immunotherapy.