Patients who leave the ED without being seen (LWBS) are unlikely to be satisfied with the quality of the service provided and might be at risk from conditions that have not been assessed or treated. We therefore examined the available research literature to inform the following questions: (i) In patients who attend for ED care, what factors are associated with the decision to LWBS? (ii) In patients who attend for ED care, are there adverse health outcomes associated with the decision to LWBS? (iii) Which interventions have been used to try to reduce the number of patients who attend for ED care and LWBS? From the available literature, there was insufficient evidence to draw firm conclusions; however, the literature does suggest that patients who LWBS have conditions of lower urgency and lower acuity, are more likely to be male and younger, and are likely to identify prolonged waiting times as a central concern. LWBS patients generally have very low rates of subsequent admission, and reports of serious adverse events are rare. Many LWBS patients go on to seek alternative medical attention, and they might have higher rates of ongoing symptoms at follow-up. Further research is recommended to include comprehensive cohort or well-designed case–control studies. These studies should assess a wide range of related factors, including patient, hospital and other relevant factors. They should compare outcomes for groups of LWBS patients with those who wait and should include cross-sectoral data mapping to truly detect re-attendance and admission rates.