Water security is key to achieving the United Nations' Sustainable Development Goals (SDGs). Yet, increasingly the world is facing water shortages, and an estimated four billion people do not have sufficient access to safe and reliable water. The link between forests and climate is regularly considered in decision-making, whereas that between forests and water remains under-represented. This report entitled "Forest and Water on a Changing Planet: Vulnerability, Adaptation and Governance Opportunities" constitutes the most comprehensive systematic scientific syntheses on the interactions between forests and water on the global level to date. Presenting the results of the sixth global scientific assessment undertaken in the framework of GFEP, the report provides a structured synthesis on the state of the knowledge on the forest-water relationship. More than 50 scientists from 20 countries contributed to this major assessment of the climate-forests-water-people link, contextually shaped by the 17 Sustainable Development Goals. This assessment report and the accompanying policy brief, launched at the 2018 UN High-Level Political Forum on Sustainable Development (HLPF 2018), provide an authoritative source of information for policymakers and stakeholders, to support effective implementation of the forest-water policies.