This report describes some of the National Institute of Justice's (NIJ) successful research projects, summarizes their findings, and discusses their influence on criminal justice policy and practice. The study considers whether the research has (1) addressed issues and problems that are central and critical to criminal justice policy and practice, (2) affected law enforcement and other functions of the system, and (3) ultimately helped improve the system's effectiveness in dealing with crime and criminals. The author asked NIJ staff to nominate research efforts that, in their opinion, had influenced policy and practice in the last two decades and then interviewed high-level managers and policymakers about their knowledge and use of the research. Finally, she reviewed the relevant criminal justice literature to see whether the changes in the field reflected or paralleled directions suggested by the research. The study indicates that research has indeed helped shape the way criminal justice policymakers and practitioners think about issues, how they identify problems that need attention, which alternatives they consider for dealing with their problems, and their sense of what can be accomplished.