There is increased interest in documenting the characteristics and treatment outcomes of clients served with Alcohol, Drug Abuse, and Mental Health Block Grant funds. The evolution of federal client-based management systems for substance abuse treatment services demonstrates that data collection systems are important but require continued support. A review of the Massachusetts substance abuse management information system illustrates the utility of a client-based data set. The development and implementation of a comprehensive information system require overcoming organizational barriers and project delays, fostering collaborative efforts among staff from diverse agencies, and employing considerable resources. In addition, the need to develop mechanisms for increasing the reliability of the data and ongoing training for the users is presented. Finally, three applications of the management information system's role in shaping policy are reviewed: developing services for special populations (communities of color, women, and pregnant substance abusers, and injection drug users), utilizing MIS data for evaluation purposes, and determining funding allocations.