Chemoprevention is a new area of research emphasis in cancer control. The rationale is based on the accumulation of laboratory and epidemiological data indicating that various agents may halt or reverse cancer progression in animals and may reduce risks in humans. For planning purposes, research leads are submitted to a strategic system of staging with defined criteria and decision points. A research lead that enters an intervention stage must evolve through a series of phases of testing and evaluation. Human intervention clinical trials have begun to test the hypothesis that certain agents can lower cancer incidence.