This systematic review assesses the extent to which primary-secondary substitution is possible in the field of emergency care where the range of options for the delivery of care is increasing in the UK and elsewhere. Thirty-four studies were located which met the review inclusion criteria, covering a range of interventions. This evidence suggested that broadening access to primary care and introducing user charges or other barriers to the hospital accident and emergency (A & E) department can reduce demand for expensive secondary care, although the relative cost-effectiveness of these interventions remains unclear. On a smaller scale, employing primary care professionals in the hospital A & E department to treat patients attending with minor illness or injury seems to be a cost-effective method of substituting primary for secondary care resources. Interventions that addressed both sides of the primary-secondary interface and recognised the importance of patient preferences in the largely demand-driven emergency service were more likely to succeed in complementing rather than duplicating existing services. The evidence on other interventions such as telephone triage, minor injuries units and general practitioner out of hours co-operatives was sparse despite the fact that these interventions are growing rapidly in the UK. Quantifying the scope for substitution in any one health system is difficult since the evidence comes from international research studies undertaken in a variety of very different health settings. Simply transferring interventions which succeed in one setting without understanding the underlying process of change is likely to result in unexpected consequences locally. Nevertheless, the review findings clearly demonstrate that shifting the balance of care is possible. It also highlights a persistent gap in professional and lay perceptions of appropriate sources of care for minor illness and injury.