Background In the context of serious concerns over the affordability of healthcare, various authors and international policy bodies advise that strategic purchasing is a key means of improving health system performance. Such advice is typically informed by theories from the economics of organization (EOO). This paper proposes that these theories are insufficient for a full understanding of strategic purchasing in healthcare, because they focus on safeguarding against poor performance and ignore the coordination and adaptation needed to improve performance. We suggest that insights from other, complementary theories are needed. Methods A realist review method was adopted involving three steps: first, drawing upon complementary theories from the EOO and inter-organizational relationships (IOR) perspectives, a theoretical interpretation framework was developed to guide the review; second, a purposive search of scholarly databases to find relevant literature addressing healthcare purchasing; and third, qualitative analysis of the selected texts and thematic synthesis of the results focusing on lessons relevant to three key policy objectives taken from the international health policy literature. Texts were included if they provided relevant empirical data and met specified standards of rigour and robustness. Results A total of 58 texts were included in the final analysis. Lessons for patient empowerment included: the need for clearly defined rights for patients and responsibilities for purchasers, and for these to be enacted through regular patient-purchaser interaction. Lessons for government stewardship included: the need for health strategy to contain specific targets to incentivise purchasers to align with national policy objectives, and for national government actors to build close, trusting relationships with purchasers to facilitate access to local knowledge about needs and priorities. Lessons for provider performance included: provider decision autonomy may drive innovation and efficient resource use, but may also create scope for opportunism, and interdependence likely to be the best power structure to incentivise collaboration needed to drive performance improvement. Conclusion Using complementary theories suggests a range of general policy lessons for strategic purchasing in healthcare, but further empirical work is needed to explore how far these lessons are a practically useful guide to policy in a variety of healthcare systems, country settings and purchasing process phases.