Many children and families experience social inequality, or the uneven distribution and allocation of social resources. Socioeconomic disadvantage includes inequalities in economic (e.g., household income, parental education, educational attainment, employment, community resources, housing, transportation, financial services) and social (e.g., social status, discrimination) areas. The Social Inequality study seeks to understand how experiencing socioeconomic disadvantage in childhood may affect the developing brain in adolescence and emerging adulthood, and how ongoing experiences of social inequality may continue to influence functioning in important domains. The Social Inequality study is a secondary data analysis project in the large, population-representative, longitudinal Adolescent Brain Cognitive Development (ABCD) Study (over 11,000 adolescents, including over 1,600 twins) and the Minnesota Center for Twin and Family Research (MCTFR) (over 1,100 now-adult twins). Understanding the effects of social inequality on substance use, depression, and other important aspects of functioning is more important now than ever, given rising income inequality in the United States and the world. This study has the potential to help us identify children and families at greatest risk, and, critically, to identify the specific social and economic factors that confer causal risk, informing the most targeted and effective prevention-intervention efforts.