Frequent emergency department (ED) users have been the targets of health care reform proposals and hospital crowding interventions, but it is not clear that common assumptions about this group are supported by data. We review the literature pertaining to frequent ED users, their demographics, acuity of illness, and patterns of health care utilization. We seek to inform development of policies directed at frequent ED use and to highlight potential related challenges. A systematic review of the literature on frequent ED users was performed. Frequent users comprise 4.5% to 8% of all ED patients but account for 21% to 28% of all visits. Most frequent ED users are white and insured; public insurance is overrepresented. Age is bimodal, with peaks in the group aged 25 to 44 years and older than 65 years. On average, these patients have higher acuity complaints and are at greater risk for hospitalization than occasional ED users. However, the opposite may be true of the highest-frequency ED users. Frequent users are also heavy users of other parts of the health care system. Only a minority of frequent ED users remain in this group long term. Complaints vary with age, location, and usage patterns. Frequent ED users are a heterogeneous group along many dimensions and defy popular assumptions. The subgroups have not yet been sufficiently defined to allow clearly directed policy design, and many frequent users present with true medical needs, which may explain why existing attempts to address the phenomena have had mixed success at best. Copyright 2009 American College of Emergency Physicians. Published by Mosby, Inc. All rights reserved.