PURPOSE: The Quebec Longitudinal Study of Child Development (QLSCD) was designed to examine the long-term associations of preschool physical, cognitive, social, and emotional development with biopsychosocial development across childhood, adolescence, and young adulthood. METHODS: QLSCD is an ongoing prospective cohort including 2120 singletons born in 1997/1998 in the Canadian province of Quebec. So far, data have been collected annually or every 2 years from child ages 5 months to 21 years. The cohort currently includes 1245 participants. Data available include a range of environmental (e.g., family characteristics, child behaviour, educational attainment, mental health), biological (e.g., hair cortisol, genetic, epigenetic), and administrative data. RESULTS: QLSCD has contributed to the understanding of children's psychosocial development, including the development of physical aggression and anxiety. QLSCD articles have advanced scientific knowledge on the influence of early childhood factors on childhood, adolescent, and young adult mental health, including the effect of participation in early childcare on cognitive and behavioural development, the developmental origins of adolescent and young adult mental health problems and suicide risk, and the development of interpersonal difficulties (e.g., peer victimisation) from preschool years to adolescence. CONCLUSION: QLSCD has given major contributions to our understanding of the link between different aspects of child development and biopsychosocial development during the first two decades of life. Unique features include the presence of environmental, biological, and administrative data, long-term follow-up with frequent data collections, and use of data from multiple informants, including teachers, mothers, fathers, and the children themselves.< Réduire