In Health Affairs, Anna Morgan and colleagues, including Judith Long, David Grande and Carolyn Cannuscio, discuss the importance of public libraries in addressing the social determinants of health and improving population health. The qualitative study describes the work of the Healthy Library Initiative, a partnership between the University of Pennsylvania and the Free Library of Philadelphia, which aims at integrating evidence-based health efforts throughout the Free Library’s fifty-four branches. The authors collected information about the Free Library’s latest programs, and conducted interviews with local residents and library staff. They find the Free Library to be highly responsive to community needs. Libraries are disproportionately frequented by vulnerable populations, including those experiencing mental illness, substance use and homelessness as well as recent immigrants and children. This broad demographic reach, as well as a geographic one, puts them in a unique position to be a safety net for vulnerable populations. In order to build on this position, the authors suggest increasing library capacity, leveraging librarians’ role as trusted community sentinels, and building cross-sectoral partnerships to advance population health. Policy makers, public health agencies, and health care systems should maximize the opportunities presented by libraries by investing in strong relationships with them and supporting them in their work by bolstering infrastructure, training and programming.