Electronic health records (EHRs) are transforming the practice of clinical medicine, but the extent to which they are being harnessed to advance public health goals remains uncertain. Data extracted from integrated EHR networks offer the potential for almost real-time determination of the health status of populations in care, for targeting interventions to vulnerable populations, and for monitoring the impact of such initiatives over time. This is especially true in ambulatory care settings, which are uniquely suited for monitoring population health indicators including risk factors and disease management indicators associated with chronic diseases. As efforts gather steam to integrate health data across delivery systems, large networks of electronic patient information are increasingly emerging. Few of the national population health surveillance systems that rely on EHR data have progressed beyond laying groundwork to launch and maintain EHR-based surveillance, but a limited number of more focused or local efforts have demonstrated innovation in population health surveillance. Common challenges include incompleteness of population coverage, lack of interoperability across data systems, and variable data quality. This review defines progress, opportunities, and challenges in using EHR data for population health surveillance.