This report examines the development over childhood and adolescence of the recorded criminal activity of two cohorts of Canadians, born in 1987 and 1990. The data are drawn from the Incident-Based Uniform Crime Reporting Survey (UCR2) for 1995 to 2005. During that period, the UCR2 received information on crime and offenders from police services in six provinces, which provided policing services to about half of the population of Canada. This is the first large-scale developmental study of delinquency in Canada based on police-reported data.