A zero-pollution ocean will only be possible if policymakers, business leaders and investors have access to sufficient evidence to evaluate the scope, scale and impact of marine pollution and use this to take action. A coordinated global response will first require building a comprehensive picture of the existing data and understanding the science needed to fill the evidence gaps. Back to Blue, an initiative of Economist Impact and The Nippon Foundation, calls on scientists, policymakers, business leaders, and investors to co-design a roadmap to close the marine pollution data gap. In this case study interview, Kenneth Leung, chair professor of environmental toxicology and chemistry at City University of Hong Kong, and director of China’s State Key Laboratory of Marine Pollution, discusses the Global Estuaries Monitoring Programme and the lessons it holds for closing the marine pollution data gap.