Underuse-the failure to use effective and affordable medical interventions-is common and responsible for substantial suffering, disability, and loss of life worldwide. Underuse occurs at every point along the treatment continuum, from populations lacking access to health care to inadequate supply of medical resources and labour, slow or partial uptake of innovations, and patients not accessing or declining them. The extent of underuse for different interventions varies by country, and is documented in countries of high, middle, and low-income, and across different types of health-care systems, payment models, and health services. Most research into underuse has focused on measuring solutions to the problem, with considerably less attention paid to its global prevalence or its consequences for patients and populations. Although focused effort and resources can overcome specific underuse problems, comparatively little is spent on work to better understand and overcome the barriers to improved uptake of effective interventions, and methods to make them affordable.