This book focuses on transitions into young adulthood by examining key roles and experiences of young adults and how they relate to changes in drug use. The findings are based on the nationwide Monitoring the Future project, and represent the majority of individuals who entered young adulthood in the United States during the years 1976 through 1994. Surveys have been conducted on them from their high school years on into their twenties and thirties. More than 33,000 participants have contributed data to this book. They have been questioned about their schooling, employment, living arrangements, marriages, pregnancies, parenthood, and divorces. Each of these aspects has then been examined for possible links with the use of tobacco, alcohol, marijuana, and cocaine. Many important differences among these drugs are documented. The impact of key role transitions on drug use was found to be much the same whether the transitions occur earlier or later in young adulthood. There are several experiences of young adulthood that seem to have the greatest impact on changes in drug use. Marital status and other aspects of living arrangements are especially important and findings suggest that marriage and pregnancy can lead to stopping or reducing the use of all four substances, which explains some of the "maturing out" or reduction in substance use among young adults in their early twenties. Other arrangements, such as leaving parents' homes and moving into dormitories or similar housing among other unrelated young adults can lead to increases in alcohol and other drug use; while life in the military leads to greater cigarette and alcohol consumption. Limitations of the study are discussed in detail in chapter three and in the extensive Appendix. The Appendix contains survey methods and procedures, multiple regression analysis, and guidelines for interpreting regression tables. (Contains 172 references, one appendix and 11 tables.) (JDM)