Over the past half-century, community health workers (CHWs) have been a growing force for extending health care and improving the health of populations. Following their introduction in the 1970s, many large-scale CHW programs declined during the 1980s, but CHW programs throughout the world more recently have seen marked growth. Research and evaluations conducted predominantly during the past two decades offer compelling evidence that CHWs are critical for helping health systems achieve their potential, regardless of a country's level of development. In low-income countries, CHWs can make major improvements in health priority areas, including reducing childhood undernutrition, improving maternal and child health, expanding access to family-planning services, and contributing to the control of HIV, malaria, and tuberculosis infections. In many middle-income countries, most notably Brazil, CHWs are key members of the health team and essential for the provision of primary health care and health promotion. In the United States, evidence indicates that CHWs can contribute to reducing the disease burden by participating in the management of hypertension, in the reduction of cardiovascular risk factors, in diabetes control, in the management of HIV infection, and in cancer screening, particularly with hard-to-reach subpopulations. This review highlights the history of CHW programs around the world and their growing importance in achieving health for all.