This systematic review describes the role and impact of physician assistants (PAs) in the ED. It includes reports of surveys, retrospective and prospective studies as well as guidelines and reviews. Seven hundred and twelve studies were identified of which only 66 were included, and many of these studies were limited by methodological quality. Generally the use of PAs in the ED is modest with 13–18% of US EDs having PAs although academic medical centres report PA use in 65–68% of EDs. The evidence indicates that PAs are reliable in assessing certain medical complaints and performing procedures, and are well accepted by ED staff and patients alike. There is limited evidence as to whether PAs improve ED flow or are cost-effective. Future studies on work processes, cost-effectiveness, unfamiliar patients' willingness to be treated by non-physician providers, and ED physicians' acceptability of PAs are needed to inform and guide the integration of PAs into EDs.