Two samples of males were compared: a representative sample of 1,684 adolescents ('adolescents') and 470 convicted youths ('delinquents'). The adolescents completed a self-administered questionnaire, which was again administered 2 years later to determine the types of delinquency in which they had participated. The delinquents were interviewed at recruitment and at intervals of 2 and 5 years. They were also administered the same self-administered questionnaire as the adolescents. A study of the five main characteristics of offending (participation, frequency, variety, gravity, and violence) provided an overall picture of the subjects' criminal activity. Findings show that criminal activity is widespread among adolescents, and it starts very early. Half the sample of delinquents had committed offenses before adolescence and persisted into adolescence. Each form of criminal activity had a distinct age for its emergence, a specific duration, and a particular age when it ceased. The study discusses criminal activity as a process of offending. 330 references, tables, charts, author index, subject index.