Health system actors look to approaches to improve poor service delivery, quality of care, and clinical outcomes that are within their capabilities and available resources. Networks are an increasingly employed approach by which to do so. This study employed a realist review approach drawing on peer reviewed and grey literature and guidance from an expert stakeholder group. Literature was selected from a previously completed scoping review on networks in low-and-middle-income country (LMIC) health systems and augmented by additional searches. This study aimed to develop an overall programme theory outlining the underlying mechanisms and interactions of contexts that explain how networks improve and provide high-quality care and services and why they work in these ways, thereby improving clinical outcomes in LMIC health systems. The review then focused more specifically on understanding how and why networks are formed and what makes them work. Our analysis suggests that this starts with the identification of a clinical or organisational problem by health systems actors who then develop a collective vision to take action around the identified problem. The process of a group of health system actors coming together around a shared collective vision leads to the development of a professional culture. When this occurs with the energy and effort to create committed, engaged, and motivated network members, who are linked through purposeful connections and relationships, and who identify with the network, changes in practice can follow. The findings from this realist review will enable a greater understanding of how networks are formed and work and why they work in these ways to improve and provide high-quality care and services. Putting these findings into practice can lead to better implementation of networks, thereby improving care and service delivery and subsequently clinical and health system outcomes. Organisations and governments implementing or interested in implementing a network in their health system may find these findings particularly relevant and useful.