The findings show that self-control--measured by the traits of impulsivity, preference for simple tasks, risk-seeking, preference for physical activity, self-centeredness, and volatile temper--was a strong and stable predictor of delinquency and substance use. The lower the level of self-control, the higher was involvement in delinquency and substance use. Of the measured components of self-control, risk-seeking and, to a lesser extent, impulsivity were almost as powerful as self-control as a total measure in predicting the outcomes. Data for the study were obtained in 1999 from a representative, stratified random sample of 2,693 ninth-grade students in the Swiss canton of Zurich, a predominantly urban and suburban area. The mean age of the students was 15.7. The six traits of self-control were measured with an adapted version of Grasmick et al.'s (1993) original scale. Delinquency was measured as the 12-month incidence of 14 common forms of juvenile delinquency. Substance use was measured as the use of nine common types of psychoactive substances over the last 12 months. 6 tables and 48 references