PROJECT SUMMARY/ABSTRACT BACKGROUND: During the first six months of 2020, 10 million Medicare beneficiaries used telemedicine services compared to only 134,000 during the first six months of 2019. Telemedicine policy changes during the COVID-19 pandemic catalyzed a major transformation of ambulatory care delivery that is sustained—currently 1 out of 5 ambulatory care visits are performed through telemedicine. While state and federal agencies have long positioned telemedicine as a means to improve health, post-pandemic telemedicine policymaking is hindered due to the scarcity of evidence on the broad scale impact of telemedicine on healthcare access, quality and costs. For instance, it is challenging to enact permanent telemedicine policies that facilitate the equitable use of telemedicine without an understanding of patient, provider and market barriers that impede its use. Furthermore, while widespread telemedicine use has the potential to improve clinical outcomes such as preventable hospital admission rates, there is a lack of empirical data on this topic. Finally, while essential for policymaking, the broad scale impact of telemedicine use on Medicare spending and utilization has not been previously studied. The dramatic increase in telemedicine use as a result of healthcare policies enacted during the COVID-19 pandemic provides an unprecedented opportunity to address these long-standing knowledge gaps. OVERALL STRATEGY: In this study, we propose using a contemporary, national cohort of Medicare beneficiaries supplemented with detailed provider and market-level data to investigate factors associated with telemedicine use, and the impact of telemedicine on ambulatory care outcomes and healthcare spending. RESEARCH AIMS: 1) To identify patient, provider and market-level determinants of telemedicine use. We will explore heterogeneity in telemedicine use across specific populations of interest, including beneficiaries that are rurally located, elderly, racial/ethnic minorities, or socioeconomically disadvantaged. 2) To evaluate the association between practice-level telemedicine use and hospital admissions for ambulatory care sensitive conditions. 3) To evaluate the impact of clinician-level telemedicine use on 30-day total per capita costs. IMPACT: Findings from this study will directly inform major areas of uncertainty and provide key evidence to inform telemedicine policymaking.