The overall objective of the study is to contribute to the understanding of the determinants of adolescents’ mental (ill-)health and social development during adolescence and young adulthood, as well as the mechanisms underlying the associations between determinants and these outcomes. A particular aim is to focus on the interplay between individual characteristics and environmental factors. In addition to this major objective, a number of participants investigate the course and determinants of somatic (ill-)health, covering topics such as overweight, lung disease, sports, physical activity, and more.The clinical cohort follows children who contacted specialty mental health services before the age of 10 years.It collects largely the same data at the same ages as the population cohort. Data collection of the clinical cohort began one assessment wave after the data collection of the population cohort.