Americans rely on emergency departments (EDs) to play a critical role in the health care safety net. But while the number of ED visits climbed significantly, the number of EDs dropped from 5,000 to 4,000 from 1991 to 2006. This synthesis examines the evidence on who uses EDs, the causes and consequences of overcrowding, and the cost of providing care in the ED. Key findings include: privately insured patients account for the largest and fastest growing segment of ED users. Low income and poor health are strong predictors on ED use. After adjusting for health status and other demographic and market factors, the uninsured use the ED at the same rate as the privately insured, while patients with Medicaid/SCHIP or Medicare had much higher utilization rates. The use of EDs for non-urgent, preventable conditions is growing due to limited access to primary care providers, although the identification of these conditions is imprecise. Patient boarding, or the inability to move patients from the ED to the appropriate inpatient unit, and a shortage of on-call specialists are major causes of ED overcrowding. The true cost of providing care in the ED is complex and not well understood. Finally, expansion of health insurance coverage is likely to increase rather than decrease stress on overcrowded EDs because the insured represent the fastest growing subpopulation of ED users.