The authors conducted a literature review on the role of the private sector in low- and middle-income countries. The review indicated that relatively few studies have researched the role of the private sector in immunization service delivery in these countries. The studies suggest that the private sector is playing different roles and functions according to economic development levels, the governance structure and the general presence of the private sector in the health sector. In some countries, generally low-income countries, the private for-profit sector is contributing to immunization service delivery and helping to improve access to traditional EPI vaccines. In other countries, particularly middle-income countries, the private for-profit sector often acts to facilitate early adoption of new vaccines and technologies before introduction and generalization by the public sector. The not-for-profit sector plays an important role in extending access to traditional EPI vaccines, particularly in low-income countries. Not-for-profit facilities are situated in rural as well as urban areas and are more likely to be coordinated with public services than the private for-profit sector. Although numerous studies on non-governmental organizations (NGOs) suggest that the extent of NGO provision of immunization services in low- and middle-income countries is substantial, the contribution of this sector is poorly documented, leading to a lack of recognition of its role at national and global levels. Studies on quality of immunization service provision at private health facilities suggest that it is sometimes inadequate and needs to be monitored. Although some articles on public-private collaboration exist, little was found on the extent to which governments are effectively interacting with and regulating the private sector. The review revealed many geographical and thematic gaps in the literature on the role and regulation of the private sector in the delivery of immunization services in low- and middle-income countries.