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National studies of treatment effectiveness in the United States during the past 20 years have influenced drug policy decisions about treatment funding, the development of new services components, and evaluation methodologies. The Drug Abuse Reporting Program was the first national evaluation study of community-based treatment programs, implemented in the early 1970s. It evolved into a 20-year longitudinal research program that included during-treatment assessments of client performance, evaluations of treatment effectiveness based on outcomes in the years immediately following treatment, and long-term studies of addiction careers. The Treatment Outcome Prospective Study began in the early 1980s and is now being followed by the Drug Abuse Treatment Outcome Study in the 1990s. These multisite and multimodality studies have helped establish broad-based evidence for treatment efficacy and directions for new studies in the United States.