Designing complex health system interventions: an integrated theory-informed approach.

Health systems interventions are increasingly designed to address complex challenges in quality, safety, access, and equity. Yet many fail to achieve sustained impact, often due to misalignment between intervention mechanisms, organisational context, and broader system dynamics. Although theory is widely promoted to strengthen intervention design, frameworks from behavioural science, implementation science, and systems thinking are frequently applied in isolation, limiting their collective explanatory and practical value. This focused narrative review adopts a design-first perspective-defined as the prospective use of theory to inform intervention development rather than its retrospective use to explain outcomes, to synthesise and compare key theoretical approaches used to inform the development of complex health system interventions. Drawing on a narrative synthesis of 46 review articles (33 identified through database searching and 13 through citation chasing of foundational sources) published between 2010 and 2026, we examine how behavioural science, implementation science theories and frameworks, and systems thinking approaches contribute complementary insights across the micro- (individual/interpersonal), meso- (team/organisational) and macro- (system/policy) levels of complexity. We argue that effective intervention development requires a layered design architecture that explicitly aligns micro-level behavioural mechanisms, meso-level organisational and implementation processes, and macro-level system dynamics. We also argue that the boundaries between these traditions are increasingly fluid, and that hybrid frameworks and the integration of design thinking with implementation and systems science, represent a particularly fertile area for advancing intervention design. By integrating these traditions, we propose a pluralistic and structured approach to intervention design that strengthens causal clarity while accommodating adaptation and contextual variability. Advancing such integrative practice is essential for improving the effectiveness, scalability, and sustainability of health system interventions.