Environmental flows are central to equitable distribution of and access to water and services provided by aquatic ecosystems. They refer to the quality, quantity, and timing of water flows required maintaining the components, functions, processes and resilience of aquatic ecosystems that provide goods and services to people. They are fundamental for sustainable water resources development, benefits sharing, and poverty alleviation. Nonetheless, water resources development planners have often overlooked environmental flows or addressed them inadequately. This two-part report, based on the Bank's economic sector analysis, shows their central importance for Integrated Water Resource Management (IWRM) and presents data, findings, and recommendations, based on in-depth case study analysis, to help make environmental water allocation an integral part of IWRM.