Cancer chemoprevention can be defined as prevention of cancer by the administration of one or more chemical entities, either as individual drugs or as naturally occurring constituents of the diet. Based largely on the time period that chemopreventive agents exhibit activity in animal models of carcinogenesis, they can be classified as inhibitors of carcinogen formation, blocking agents, and suppressing agents. The majority of compounds that inhibit the formation of carcinogens prevent the formation of nitrosamines from secondary amines and nitrite in an acidic environment. Blocking agents are inhibitors of tumor initiation, while suppressing agents are inhibitors of tumor promotion/progression. Many well-characterized chemopreventive agents act at one or more steps in both tumor initiation and promotion/progression. The objective of this paper is to provide a general discussion of the mechanisms through which chemopreventive agents inhibit carcinogenesis. Examples of agents that act through these mechanisms are given; however, a complete listing of effective chemopreventive agents is not possible within the context of this paper. At the conclusion is a brief discussion of future prospects in cancer chemoprevention and obstacles to overcome.