Data were collected by personal interviews based on a structured instrument. Delinquency was measured by self-report questions on seven typical youth offenses (property and violent). Questions were added on drug use, truancy, and running away from home, acts not considered offenses in Dutch juvenile penal law. A second test was performed 2 years later on a subsample of the original one, taking into account changes in the life situation with respect to the family, school, work, leisure, peers, and values associated with delinquency. Changes in social integration were measured over time and compared to the impact of official intervention in delinquency cases by police or prosecutors. Results demonstrated no impact of police or prosecutor intervention on later delinquency but did show a strong and inverse relation between changes in social integration and frequency of offending. Although delinquent peers approved of and supported delinquent behavior, juveniles did not join these groups by chance. Those who failed in conventional subsystems and did not get any rewards from them turned away from conventional groups and toward other marginal and deviant peers. This process was accelerated by such factors as truancy, spending leisure time in the streets, and running away. The best predictor of delinquency was not poor family functioning, but rather failure in school. The author concludes that the contribution of social control theory to understanding delinquent behavior is a major one. Research questions are contained in an appendix. 19 references, 6 tables, and 4 figures