Spread, Scale, and Sustainability in Population Health is the summary of a workshop convened by the Institute of Medicine's Roundtable on Population Health Improvement in December 2014 to discuss the spread, scale, and sustainability of practices, models, and interventions for improving health in a variety of inter-organizational and geographical contexts. This report explores how users measure whether their strategies of spread and scale have been effective and discusses how to increase the focus on spread and scale in population health.