ABSTRACT Disparities in health within the U.S. are pervasive and, for some populations, widening. High-quality primary care plays an important role in the prevention, diagnosis and management of the many chronic health conditions that contribute to health disparities among older adults. Primary care in the U.S., however, is threatened. Even before COVID-19, the per-capita supply of primary care physicians was falling and varied dramatically by county, threatening rural and other less-advantaged communities. Little is known about how access to high-quality primary care has changed in recent years—including changes in access associated with the COVID-19 pandemic—or about the policy-, system-, or practice-level factors that are associated with better quality of primary care for older adults. These gaps in understanding have hindered our ability, as a nation, to provide the best care to older adults. This project will address this need by drawing on a unique national dataset that includes annual information on the ownership and staffing of all U.S. primary care practices from 2015–2024, linked Medicare claims data, and surveys of nationally representative samples of these practices conducted in 2017 and 2022. Work in this project will entail: Aim 1: Examine U.S. trends in access to primary care for Medicare beneficiaries in traditional Fee-for-Service and Medicare Advantage and determine how these trends varied for less-advantaged populations. The team will conduct repeated cross-sectional studies of access to primary and relevant subspecialty care for Medicare enrollees and how trends in access to care varied for less-advantaged populations. Aim 2: Identify the policy-, system-, and practice-level factors associated with better processes and outcomes of care for Medicare beneficiaries, with a focus on those with fewer social and economic advantages. The team will take advantage of the substantial differences across states, delivery systems, and physician practices in the implementation of initiatives intended to improve and support primary care to apply differences-in-differences approaches to identify potentially high impact factors. Aim 3: Conduct qualitative research to deepen our understanding of the underlying barriers and facilitators to improving primary care for less advantaged populations. Under this aim, the team will conduct key informant interviews with experts on policy, primary care and the safety net to deepen our understanding of current challenges and opportunities facing safety net practices. The team will then purposively sample practices that participated in the 2022 survey that serve economically less-advantaged and minoritized populations and conduct in-depth qualitative interviews with their leaders and staff. Findings across these three aims will be triangulated to develop recommendations that can assist practice leaders, health system leaders, and policymakers in improving primary care and reducing health disparities for old...