Carcinogenesis is the multistep process by which normal cells undergo malignant transformation. The clinical expression of cancer may be prevented or delayed either by risk factor modification, such as quitting smoking, or by the administration of drugs to prevent or delay the clinical expression of the malignant phenotype. This chapter focuses on the latter approach, often referred to as ˵chemoprevention.″ Specifically, it addresses the two key issues distinguishing cancer prevention from cancer treatment agent development: selection of study endpoints and patient cohorts. In addition, a comprehensive update is provided of recently completed and ongoing phase 1, 2, and 3 prostate cancer prevention clinical trials conducted under the auspices of the Division of Cancer Prevention, NCI. [ABSTRACT FROM AUTHOR]