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.