This review focuses on frequent users (FUs) of the emergency department (ED). Elucidation of the characteristics of frequent ED users will help to improve healthcare services. A systematic review of the literature (from 1999 onwards) on frequent ED users in non-US hospitals was performed. Twenty-two studies were included. FUs are responsible for a wide variety of 1-31% of ED visits depending on the FU definition used. They have a mean age between 40 and 50 years and are older than nonfrequent users. Chronic physical and mental diseases seem to be the main reasons for frequent ED visits. In terms of social characteristics, lacking a partner is more frequently reported among FUs in some studies. The absence of a universal definition for FUs complicates the determination of the burden on emergency healthcare services. FUs are a heterogeneous group of patients with genuine medical needs and high consumption of other healthcare services.