OBJECTIVES: The US armed forces adopted "zero tolerance" policies concerning illicit drug use in 1980 and later developed policies to discourage tobacco and alcohol abuse. This article examines drug use among young active-duty recruits both before and after enlistment, compared with nonmilitary age-mates, and documents historical shifts in such drug use across 2 decades. METHODS: Analyses employed longitudinal panel data from 20 nationally representative samples of high school seniors (cohorts of 1976-1995), each surveyed just before graduation and again within 2 years. Separate analyses for men (n = 12,082) and women (n = 15,345) contrasted those who entered military service, college, and civilian employment. RESULTS: Illicit drug use declined more among young military recruits than among their civilian counterparts. Analyses of male recruits at multiple time periods showed (1) declines in the prevalence of marijuana use and cocaine use after the initiation of routine military drug testing and (2) lower proportions of smokers of half a pack or more of cigarettes per day who entered service after the initiation of tobacco bans during basic training. CONCLUSIONS: Recent military drug policies appear to deter illicit drug use among enlistees and discourage some smokers from enlisting.