Introduction.

The Adolescent Brain Cognitive Development (ABCD) Study is a longitudinal, observational study of over 10,000 youth recruited at 21 sites throughout the United States. Comprehensive biennial assessments and more limited interim assessments measure health, mental health, neurocognition, family, cultural and environmental variables, substance use, genetic and other biomarkers, and structural and functional brain development. Within this Special Issue, readers will find much information about the rationale and objectives of the study, the broad ranging assessment protocols and new as well as traditional methodologies applied at baseline, the recruitment and retention strategies, and the anticipated final composition of the cohort. Information is also provided about how the study is coordinated and conducted, how decisions are made, how data quality is monitored, and how ethical standards are protected. In this introduction we will focus instead on the position of the ABCD Study in the changing landscape of biomedical research.