Theory of systems change: An initial, middle-range theory of public health research impact.

This article presents the Theory of Systems Change, a new middle-range theory developed to explain how public health research that applies a systems thinking perspective contributes to changes in system behavior and improved population health outcomes. The theory identifies well-functioning systems as those characterized by four interrelated practices—evidence-driven action and learning, adaptation, alignment with target population needs, and collaboration—underpinned by embedded capacity across research, practice, and public policy domains. Public health research contributes to system change primarily by embedding capacity through involving practitioners and policy-makers in knowledge production and disseminating credible, accessible, relevant, and scalable research knowledge. Developed and applied within the Pathways in Place research program in Australia, the theory aims to guide future research evaluation and advance understanding of research impact in complex public health systems, while acknowledging its initial and testable nature.