This report is based on the results of the National Drug/Alcohol Collaborative Project which examined the prevalence of multiple substance use, particularly the combined use of drug and alcohol. The introductory chapter describes the background and purpose of the project. Chapter Two reviews the literature about multiple substance abuse and discusses conceptual and historical definitions of multiple substance-taking behavior, medical hazards, and drug use patterns of alcoholics as well as alcohol use patterns of drug addicts. Chapter Three presents findings of substance use patterns identified by the 10 project programs, as well as users' consequences, demographic and psychosocial characteristics of the sample, and descriptions of the participating programs. Chapter Four discusses the findings of an experimental study to examine the efficacy of treating drug and/or alcohol abusers separately and together. The final chapter presents the methodology and findings of an additional study that examined the nature and extent of alcohol use and treatment outcomes in a drug-abusing sample. (Author/NRB)