The optimal way of dealing with any disease is by prevention. This is particularly true of cancer, with all of its complexities. A major problem that exists for cancer prevention is that we do not know the cause of over 50% of cancers. Even when causes are known, serious difficulties often exist in removing them. To the extent that causality cannot be dealt with effectively, other strategies merit consideration. One is chemoprevention. A great strength of chemoprevention is that a large number of compounds can prevent the occurrence of cancer, and a variety of mechanisms exist for producing such protection. Much of this review deals with efficacy, toxicity, and mechanisms of action of chemopreventive agents. Gap areas in information are discussed, as well as opportunities for producing compounds with optimal attributes. Chemoprevention is not simple, and successes may not come quickly. However, for both the general population and, even more urgently, for individuals at high risk, chemoprevention has the potential of providing an important means for cancer prevention.