The volume recognizes that much social science research must contend with a variety of definitional issues. This problem is especially acute for research that relies on official records, but it is a problem that affects all scientific research since science is by definition a cumulative enterprise. Data sources do not automatically provide standard measures of events, activities, attitudes, or behaviors, and official records vary significantly across jurisdictions. In addition to problems with official records and reporting, researchers face definitional issues in categorizing for common use the behaviors they are attempting to study. Contributors to the volume pose a variety of questions about youth and justice, describe contexts in which young people encounter the justice system, and lay out the core of existing research on key topics concerning youth and justice.