The Roundtable on Population Health Improvement of the National Academies of Sciences, Engineering, and Medicine hosted a public workshop on September 19, 2019 titled Models for Population Health Improvement by Health Care Systems and Partners: Tensions and Promise on the Path Upstream. The term upstream refers to the higher levels of action to improve health. Medical services act downstream (i.e., at the patient level) in improving population health, while such activities as screening and referring to social and human services (e.g., for housing, food assistance) are situated midstream, and the work of changing laws, policies, and regulations (e.g., toward affordable housing, expanding healthy food access) to improve the community conditions for health represents upstream action.The workshop explored the growing attention on population health, from health care delivery and health insurance organizations to the social determinants of health and their individual-level manifestation as health-related social needs, such as patients’ needs. The workshop showcased collaborative population health improvement efforts, each of which included one or more health systems. This publication summarizes the presentations and discussions from the workshop.