Conducted as part of the Monitoring the Future project, this study used a cohort-sequential design to examine period, age, and cohort effects on substance use among American youth between the ages of 18 and 28 from the high school classes of 1976 to 1986. This manuscript supersedes Paper 14 in the series which reported on American youth from 18-24 years old from the classes of 1976 to 1982. A total of 18 variables were analyzed, dealing with 12 different drug classes. The results revealed several different types of period effects over the past decade. Period effects in the form of monotonic decreases occurred for annual prevalences of barbiturates, some psychedelics, and tranquilizers. Bilinear period effects, first increasing then decreasing, were found for marijuana, amphetamines, lysergic acid diethylamide, and a measure of heavy drinking. Cocaine, methaqualone, and monthly prevalence of alcohol use all reflected complex period effects. Effects of age were also complex and varied across drugs. Cigarette use increased after high school until age 21, after which it decreased. Monthly and daily alcohol use increased linearly until age 21, and then stayed constant. A measure of heavy drinking showed a curvilinear age trend, increasing through age 21 and then decreasing, and marijuana use exhibited a similar age effect. Clear cohort (or class) effects appeared for cigarette use, with each successive class smoking less at all levels. A negative linear class effect also appeared for daily marijuana use. (Author/NB)