As the United States seeks to improve the value of health care, there is an urgent need to develop quality measurement for emergency departments (EDs). EDs provide 130 million patient visits per year and are involved in half of all hospital admissions. Efforts to measure ED quality are in their infancy, focusing on a small set of conditions and timeliness measures, such as waiting times and length-of-stay. We review the history of ED quality measurement, identify policy levers for implementing performance measures, and propose a measurement agenda. Initial priorities include measures of effective care for serious conditions that are commonly seen in EDs, such as trauma; measures of efficient use of resources, such as high-cost imaging and hospital admission; and measures of diagnostic accuracy. More research is needed to support the development of measures of care coordination and regionalization and the episode cost of ED care. Policy makers can advance quality improvement in ED care by asking ED researchers and organizations to accelerate the development of quality measures of ED care and incorporating the measures into programs that publicly report on quality of care and incentive-based payment systems.T he care delivered in hospital-based emergency departments (EDs) is an important element of the US struggle to improve access to and quality of health care.1 EDs are the critical staging area for severely ill patients, the site of one in eleven ambulatory care visits.2 EDs play a key role in half of hospital admissions. And EDs are an important part of America's safety net, because they are required by law to evaluate all patients, regardless of ability to pay. 3 In 2010 there were 130 million ED visits in the United States, and yearly increases in ED visits have consistently outpaced population growth. 4 Providers, payers, and the general public have differing views of the quality of ED care. For example, payers often classify the ED as overused and costly, while emergency medicine specialty societies consider ED care to be efficient and safe. And the increasing use of the ED by the public can be interpreted as indicating that many people see the ED as a source of highquality care.Part of the difficulty in knowing which view of ED care is correct is that ED quality measurement today is incomplete. Care for certain conditions, such as acute myocardial infarction (heart attack), is closely monitored, performance is publicly reported, and high-quality care is rewarded-all of which creates a strong incentive to improve. But the majority of care delivered in EDs goes largely unmeasured, with the exception of metrics for length-of-stay and waiting times.Rigorous quality measurement in health care is still a relatively new field. The focus to date has been on developing measures for specific conditions, such as congestive heart failure, instead of