Discussions of the juvenile justice system cannot proceed without a clear understanding of the behaviors that it is tasked to address. This chapter looks at the varying definitions of delinquency and who is a juvenile in different jurisdictions. The chapter examines how delinquency is measured using official police, court, and correctional data and self-report data. The measures are examined for their strengths, weaknesses, and uses. The great discrepancies between the measures are discussed.