The Youth and Health study is part of the international Health Behaviour in School-aged Children (HBSC) study and is carried out in collaboration with the World Health Organization (WHO/EURO). Every four years, HBSC collects data on health (mental and physical health), health behaviours (physical activity and sedentary behaviour, nutrition, overweight and obesity, sexual health and alcohol, tobacco and drug use) and the social environment (school, family and friends) of 11-, 13- and 15-year-old adolescents in approximately 50 European and North American countries and regions. The international HBSC study was established in 1982 and Flanders participated for the first time in 1989. The aim of the study is to gain more insight into health-related attitudes and behaviours of adolescents and of the environment in which they develop. In addition, the study wants to map the evolutions in these attitudes and behaviours. As such, the study intends to provide policymakers, scientists and actors working in the field of health promotion with valuable information to develop and adjust initiatives to promote adolescents’ health. Finally, the international context - with its broad network of experts -stimulates further development of the field of adolescent health promotion and allows monitoring and cross-national comparison of evolutions in adolescents’ health and health behaviours. In Flanders, a representative sample of adolescents from 5th primary education to 6thsecondary education is drawn (N = 11 000, 10- to 18-year olds).