PROJECT SUMMARY Adverse birth outcomes, including prematurity and low birthweight, are a persistent public health problem in the United States. They affect nearly half a million newborns each year and contribute to long-term health challenges like cardiovascular disease and cognitive impairment. Risk for adverse birth outcomes is differentially distributed across the population and disproportionately high in low-income households. Past research suggests this discrepancy at birth is at least partially attributable to differing social and environmental exposures during the prenatal period, including differences in housing stability and affordability. Housing insecurity, or limited access to safe, affordable housing, is a common material hardship in low- income households, forcing many into unstable housing conditions and making health-related needs, like food and healthcare, more difficult to access and afford. Pregnancy is an especially vulnerable time for households to experience housing insecurity given the sensitivity of the mother and the developing fetus to environmental stress. A growing body of evidence links prenatal experiences of housing insecurity to adverse birth outcomes. For decades, the flagship policy solution to housing insecurity in the U.S. has been federal housing assistance. Administered by the U.S. Department of Housing and Urban Development (HUD), housing assistance programs provide stable, affordable housing to nearly 10 million individuals each year, including 5 million families. By reducing exposure to housing insecurity, housing assistance may play a critical role in reducing risk of adverse birth outcomes. Few studies, however, have investigated the impact of housing assistance on neonatal health and current estimates of an association with children's health are mixed, with roughly half of the analyses indicating no association and a quarter indicating negative health impacts. Many scholars attribute this inconclusive evidence base to a persistent set of methodological and empirical limitations, including selection effects, measurement error, effect heterogeneity, and mechanism ambiguity. To address these research gaps, our proposed study seeks to evaluate the impact of prenatal housing assistance on risk for adverse birth outcomes using a novel linkage of HUD administrative records for 2002- 2021 with California birth certificate data for 2004-2019. Our specific aims are the following: (1) To estimate the effect of prenatal housing assistance on birth outcomes, using quasi-experimental methods; (2) To determine if the effect of housing assistance on birth outcomes varies according to (a) category of assistance or (b) prenatal timing of assistance; and (3) To explore the role of neighborhood quality and maternal health behavior in mediating the relationship between prenatal housing assistance and birth outcomes. As government and health systems increasingly invest in the "promise" of affordable housing as preventive car...