PROJECT SUMMARY/ABSTRACT Substance use, particularly opioid use, during pregnancy is widespread and associated with adverse outcomes for the pregnant individual and the developing child. Prenatal substance exposure is associated with a wide range of negative fetal and child outcomes including reduced fetal growth, premature birth, lower birth weight, congenital defects, increased neonatal healthcare, and heightened risk for later behavioral (e.g., anxiety, inattention), cognitive (e.g., memory deficits, delayed language acquisition), and metabolic problems. It is difficult to understand these strong links to adverse maternal, fetal, and child outcomes, without gathering a complete picture of the other contextual factors and exposures that may contribute in a diverse and representative sample from across the US. The HEALthy Brain and Child Development (HBCD) study is a 25-site longitudinal prospective study of early child development in the US that will assess a broad spectrum of biological (e.g., neuroimaging, genetics, epigenetics), behavioral (e.g., cognition and emotional regulation), experiential (e.g., trauma), social (e.g., racism), and health (e.g., psychopathology) factors among ~7,500 nationally-representative pregnant women and their children from pregnancy to mid-childhood. A major goal of the HBCD study is to increase understanding of the potential consequences of prenatal substance exposures. It will be enriched for maternal substance use during pregnancy (i.e., ~25% of the sample will be using opioids, cannabis, alcohol, and/or tobacco during pregnancy and 12% of the total sample will be using opioids) and offers a unique opportunity to inform our understanding of how the adverse consequences associated with opioid use during pregnancy arise. It is absolutely imperative that the populations recruited across sites be representative of the US birthing population to ensure generalizable translation of the findings to be generated. This Diversity Supplement in response to the NOSI to Encourage Eligible NIH HEAL Initiative Awardees to Apply for PA-20-222: Research Supplements to Promote Diversity in Health-Related Research (NOT-NS-20-107) proposes to leverage the HBCD study to better understand the impact of recruitment and retention strategies at the site level, combined with site personnel structure, to optimize recruitment and retention of the vulnerable populations enrolled. The unique combination of a standardized protocol being implemented across the US with the proposed qualitative approaches will provide invaluable information for both optimizing recruitment as part of HBCD, as well as informing future studies, leading to more equitable and inclusive science for all people.