Adolescence is a time of extraordinary physical, emotional, and intellectual growth. The Adolescent Brain Cognitive Development (ABCD) Study was designed to better understand the many childhood experiences that shape brain, social, emotional, and intellectual development during adolescence. The ABCD Study is the largest longitudinal study of brain development and child health in the U.S., having enrolled nearly 12,000 youth starting at ages 9-10 who will be followed for a decade. Youth participants undergo magnetic resonance imaging to assess brain structure and function, provide biospecimens for pubertal hormone, substance use, and genetic analyses, and take part in a range of behavioral and neurocognitive assessments. Both youth and their caregivers will provide information about physical and mental health, culture and environment, and other variables. These measures include questions about attitudes, experiences, and behaviors that may influence risk and resilience, as well as cognitive tasks that tap into the propensity of an individual to engage in such behaviors. Such a comprehensive array of questions provides the opportunity for researchers to tease apart the factors that tip the scale towards unhealthy risk behaviors and to identify those factors that can restore a healthy balance. ABCD data is released to the scientific community through the National Institute of Mental Health Data Archive on a regular basis, allowing scientists worldwide to conduct analyses, pool resources, and enrich the value of this study, with the ultimate goal of providing actionable information to help educators, health professionals and policymakers improve the lives of all children, today and for generations to come.