This provocative Element is on the 'state of the art' of theories that highlight policymaking complexity. It explains complexity in a way that is simple enough to understand and use. The primary audience is policy scholars seeking a single authoritative guide to studies of 'multi-centric policymaking'. It synthesises this literature to build a research agenda on the following questions: 1. How can we best explain the ways in which many policymaking 'centres' interact to produce policy? 2. How should we research multi-centric policymaking? 3. How can we hold policymakers to account in a multi-centric system? 4. How can people engage effectively to influence policy in a multi-centric system? However, by focusing on simple exposition and limiting jargon, Paul Cairney, Tanya Heikkila, Matthew Wood also speak to a far wider audience of practitioners, students, and new researchers seeking a straightforward introduction to policy theory and its practical lessons.