The ABCD (Adolescent Brain Cognitive Development) Study is an ongoing longitudinal research study being undertaken by a consortium of 21 data collection sites across the United States as well as a Coordinating Center and a Data Analysis Informatics & resource Center. It is the largest long-term study of brain development and child health in the U.S. The study collects data on behavior and brain development of nearly 12,000 youth beginning at ages 9-10 and continuing through young adulthood in six domains: physical health, mental health, neurocognition, substance use, novel technology (e.g., Fitbit, app usage), and family, friends & community, as well as neuroimaging (structural and functional MRI and diffusion MRI), and biospecimens collection to measure environmental exposures and substance use. Data derived from linked external datasets using geocoded residential and school histories enrich the study’s data collection by providing information about the contexts and environments in which youth live, work, and play.