CATS is an ongoing Melbourne-based longitudinal study following a cohort of nearly 1,200 young people as they transition from childhood through adolescence and into young adulthood. The cohort of children were recruited at age 8–9 years old (whilst in grade 3 children; the fourth year of formal schooling in Australia) in 2012 from a sample of schools in metropolitan Melbourne, Australia. In total, around 1,200 children and their parents/guardians enrolled and took part in wave 1 data collection. The cohort of young people are followed-up annually through parent, teacher and self-report questionnaires.