BACKGROUND There is a growing interest in the role of private health providers in low- and middle-income countries (LMICs). Informal private providers (IPPs) provide a significant portion of health care in many LMICs, but they have not received training in allopathic medicine. Interventions have been developed to take advantage of their potential to expand access to essential health services, although their success is not well measured. This paper addresses this information gap through a review of interventions designed to improve the quality, coverage, or costs of health services provided by IPPs in LMICs. METHODS A search for published literature in the last 15 years for any intervention dealing with IPPs in a LMIC, where at least one outcome was measured, was conducted through electronic databases PubMed and Global Health, as well as Google for grey literature from the Internet. RESULTS A total of 1272 articles were retrieved, of which 70 separate studies met inclusion criteria. The majority (70%) of outcomes measured proximate indicators such as provider knowledge (61% were positive) and behaviour (56% positive). Training IPPs was the most common intervention tested (77% of studies), but the more effective strategies did not involve training alone. Interventions that changed the institutional relationships and contributed to changing the incentives and accountability environment were most successful, and often required combinations of interventions. CONCLUSION Although there are documented interventions among IPPs, there are few good quality studies. Strategies that change the market conditions for IPPs-by changing incentives and accountability-appear more likely to succeed than those that depend on building individual capacities of IPPs. Understanding the effectiveness of these and other strategies will also require more rigorous research designs that assess contextual factors and document outcomes over longer periods.