The aim of this chapter is to introduce the concepts and science of water for the environment, with emphasis on the methods available to assess the environmental flow requirements of rivers, and the challenges of assessing and providing environmental water regimes under scenarios of shifting climatic regimes and novel ecological baselines. The effects of flow regime alterations on riverine ecosystems are introduced and summarized in tabular form, citing global examples of typical ecological consequences. Environmental flow assessment methods are tabulated in four main categories (hydrological, hydraulic, habitat modeling and holistic ecosystem approaches), together with examples of applications at reach, river and regional scales. Shifting climates and novel ecological baselines are driving new demands for enhanced predictive capabilities, dynamic modeling of future river ecosystem states, and broader perspectives on planning and managing environmental water for the benefit of people and ecosystems. Recent activities to strengthen the position of environmental flows and freshwater ecosystem recovery in global policy arenas include: a revised definition of environmental flows, its uptake into the SDG 6.4.2 Water Stress Indicator, and efforts to firmly position environmental flows in planning for the Post-2020 Global Biodiversity Framework.