An overview of the criminal justice system and the development of its data systems is presented to elucidate how the nature of the system shapes the quality and usefulness of the information available. Inconsistencies in the units of count and definitions used in gathering data on criminal, law enforcement, court, and corrections activities are discussed. Problems and compromises in systemic analyses also are considered. Differences in defining correctional facility capacity and counting beds are discussed with reference to overcrowding. The parole and probation violation processes are described with respect to measurement issues within the system. The quality of data on jail population's difficulties in the definition and measurement of recidivism, and difficulties in making predictions and the accuracy of predictions are considered with reference to the effects of offender characteristics on decisionmaking are addressed. Major measurement issues identified include data availability, quality, accessibility, and verifiability; interagency differences in definitions, units of count, time frames, statistical techniques, and operational concerns; inadequacies in data systems; ethical issues; insufficient resources; and lack of data linkages across the system. Recommendations for improvement are made. Figures and appendixes.