Two prospective longitudinal surveys based on New York State high school students indicate well-defined steps underlying adolescent progression and regression in drug use. At least four stages of involvement with drugs can be identified: (1) beer or wine; (2) cigarettes or hard liquor; (3) marijuana; and (4) other illcit drugs. Two stages of legal drugs are necessary intermediates between nonuse and marijuana. Very few youths progress to other illicit drugs without prior experience with marijuana. This sequence is found in each year of high school in the year following graduation. Progression to a higher-ranked drug is directly related to intensity of use at the prior stage. The identification of stages in drug behavior has implications regarding the optimum strategy for studying factors that predict, differentiate, or result from drug use.