For more than two decades, a disparate range of activities that become prevalent in adolescence have been brought together and viewed collectively as 'problem behaviours'. These activities include cigarette smoking, drinking alcohol, taking illicit drugs, early sexual intercourse, as well as various forms of delinquency. Our opening chapter examines the origins of this concept, describes its later formulations, notes specifically developmental aspects of the theory and considers its role in understanding adolescent health-related behaviours.