Mental disorders account for one-fifth of the total burden of disease in the Western world, and, as such, should require due attention from the international epidemiological research community. Good quality research on the aetiology and course of psychopathology in the population is impossible without reliable and valid data from long-term longitudinal cohort studies. Research on psychopathology in adolescence is important both from a scientific point of view and from the point of view of prevention and public health policy. Adolescence is characterized by major biological, psychological and social challenges and opportunities, where interaction between the individual and environment is intense, and developmental pathways are set in motion or become established. Furthermore, adolescent psychopathology can have important consequences for education, relationships and socioeconomic achievement in later life. These characteristics of adolescence do not only set high demands for cohort studies aiming to capture the most salient aspects of developmental pathways, they also ensure a great gain in empirical knowledge and an invaluable source of information for public health policy from such studies. In order to fully benefit from this potential, a multidisciplinary approach is essential. The ‘Tracking Adolescents’ Individual Lives’ Survey’ (TRAILS) has taken such an approach. It started in 2001 in order to learn more about the aetiology and course of psychopathology in the Dutch population. Because most forms of adult psychopathology have antecedents and precursors in childhood and adolescence it was decided that a cohort of sufficient size should at least follow respondents from preadolescence up until at least early adulthood. The period from preadolescence into adulthood can furnish major insights into the causes of mental (ill-)health, (mal)adjustment and social development. Within this period there is a substantial rise in psychopathology. In The Netherlands, the 1-year prevalence of psychiatric disorders increases from about 10% at age 10–12 to 25% at age 23–25, of whom nearly half suffer from significant impairments. The TRAILS consortium is broad and multidisciplinary. It includes members of various departments, including (child and adolescent) psychiatry, epidemiology and biostatistics, social sciences, health sciences, movement sciences, pediatrics and respiratory disease across several universities throughout The Netherlands. Principle investigators are Prof. J. Ormel of the Department of Psychiatry of the University Medical Center Groningen, and Prof. F.C. Verhulst of the Department of Child and Adolescent Psychiatry of the Erasmus University Medical Center Rotterdam.