There is co-morbid occurrence of adolescent problem behaviours, and these behaviours manifest differently and vary in severity. For instance, some adolescents who suffer from stress and distress become addicted to substance and internet use, whereas some act out in the form of misbehaviour and delinquency. In particular, research findings have shown that there are gender differences; with boys tending to employ externalising and risk-taking behaviour as ventilation, while girls tend to internalise stress and become depressed. Nevertheless, different problem behaviours may share similar aetiologies. Adopting the ecological approach, adolescents' behaviours result from the complex interplay between themselves and the surrounding environment. These issues will be addressed in this book with experience from research from Hong Kong in a Chinese context.