This document discusses using systems theory to guide the implementation of Health in All Policies (HiAP). It advocates applying a systems framework to understand HiAP as an complex system with emergent and non-linear outcomes. The "win-win" approach aims to generate health gains without diminishing other policy sectors' goals, and is identified as most effective for facilitating HiAP implementation through developing shared language, using evidence, and integrating health into other agendas. A systems approach provides context for understanding HiAP's success or failure by considering relationships within and external influences on government systems.