Background Emergency department (ED) crowding is a significant international problem. There is increasing overseas research into this field. In the UK, the focus has been on waiting times in EDs, and on the government's 4 h standard, rather than on crowding itself.

Aims To examine the causes and effects of ED crowding, along with solutions. To consider whether the 4 h standard has had an effect on ED crowding in the UK.

Methods A structured literature review on ED crowding.

Results The evidence base largely consists of retrospective or descriptive studies (65% combined) from North America and Australasia (89% combined). Measurement of crowding is not well developed, and the lack of a gold standard additionally limits the quality of research. The main cause of crowding is access block, because of high levels of hospital occupancy. Crowding carries a number of adverse consequences for patients and staff. Many solutions are described, but with weak evidence behind them. Most of these focus on interventions in the ED, despite the fact that the main causes lie outside. Solutions aimed at achieving the 4 h standard may mitigate crowding.

Conclusion The extent of ED crowding in the UK is unknown. The problem is probably mitigated by process standards such as the 4 h standard. The causes and effects of crowding are likely to be the same as overseas, but there is little research to validate this. The best solutions are not known.