“Flow: The Essentials of Environmental Flows” is a guide to the implementation of environmental flows in the river basins of the world. It explains how to assess flow requirements, change the legal and financial framework, and involve stakeholders in negotiations. ‘Flow’ sets out a path from conflict over limited water resources and environmental degradation to a water management system that reduces poverty, ensures healthy rivers and shares water equitably. It is offered as part of the International Union for Conservation of Nature (IUCN) Water and Nature Initiative (WANI)’s Toolkit series, which aims to support learning on mainstreaming an ecosystems approach in water resource management. The tools are aimed at practitioners, policy-makers and students from NGOs, governments and academia. Versions in Spanish, Portuguese, French, Chinese, Khmer, Vietnamese and Lao also available.