Objective To determine the causes of emergency department (ED) crowding and to identify evidence-based solutions.

Design The review used a ‘conceptual synthesis’ approach to identify knowledge and opinion around the issue of ED crowding, not just effective interventions. Recommendations from the literature were classified according the quality of evidence and the extent to which they were under ED control.

Data sources SCOPUS and ISI were searched for studies of ‘ED’ AND ‘crowding OR overcrowding’ and backward citation retrieval was undertaken. To help identify systematic review evidence of effective interventions, the Cochrane Database, the National Institute of Health and Clinical Excellence (NICE) and NHS Evidence were searched. A Google search was included to identify relevant grey literature.

Eligibility criteria Papers were included if they added to substantive knowledge of ED crowding. Empirical studies, studies from the UK and studies of physical space were privileged in the review.

Results There is an established international literature on ED crowding. It suggests consistently that crowding has significant negative consequences. However, the literature offers limited practical help to practitioners for a number of reasons, such as a lack of shared definition and measurement of crowding and lack of evaluation of interventions. Many studies are single case studies from the USA.

Conclusions While current evidence is poor, this does not justify maintaining current practice which risks lives. Building up an evidence base is critical, but requires agreed definitions, measures and methods, which can be applied to systematic evaluation of plausible solutions.