Visits to hospital emergency departments (EDs) have increased greatly in recent years, contributing to crowded conditions and ambulance diversions. Contrary to the popular belief that uninsured people are the major cause of increased emergency department use, insured Americans accounted for most of the 16 percent increase in visits between 1996-97 and 2000-01, according to a study by the Center for Studying Health System Change (HSC). This Issue Brief examines trends in emergency department and other ambulatory care use, focusing on differences among insurance groups. Although insured people accounted for most of the increase in emergency department visits, uninsured Americans increasingly rely on emergency departments because of decreased access to other sources of primary medical care. Emergency department waiting times also have increased substantially, which may lower both insured and uninsured patients' perceptions of the quality of their care.