Objective: The purpose of this study was to examine change in substance use with marriage, premarriage similarity in substance use between spouses, and the role of spouse use in predicting changes in use with marriage. Method: Nationally representative samples of high school seniors were followed longitudinally through age 35. The sample consisted of 2,169 adults from eight senior-year cohorts (1977-1984) from the Monitoring the Future study who completed a questionnaire at least once before their first marriage and at 2-year intervals at four consecutive points in time after marriage. Results: Results indicate significant reductions in use with marriage for cigarette smoking, heavy drinking, and marijuana use. Both men and women reported reductions in all three substances following marriage. Changes in women's use followed a linear pattern. Although men's decreases in cigarette smoking and heavy drinking were initially rapid and then decelerated, their decrease in marijuana use accelerated over time. Declines in use for both genders were associated with getting married to individuals who also decreased their use. Those with higher premarriage levels of substance use experienced greater reductions in use. A significant association between respondent and spouse premarital use suggests assortative mating takes place for all three substances.; Conclusions: This study affirms and further qualifies the presence of a marriage effect on substance use using multiwave and multicohort national data. Results suggest that the effects of marriage on smoking, heavy drinking, and marijuana use are related to one's own initial levels of use and the direction of change in the spouse's use. These findings have important implications for life span theoretical advances as well as interventions aimed at the marital dyad. (J Stud. Alcohol Drugs 69: 352-361, 2008) Reprinted by permission of the publisher.