Benefits to people from water ecosystems like rivers, swamps, floodplains and groundwater systems are central to human well-being. But ecosystems are in trouble and the Millennium Ecosystem Assessment, the Comprehensive Assessment of Water Management in Agriculture, and the Intergovernmental Panel on Climate Change have each shown that freshwater ecosystem services are particularly vulnerable. Water problems for poor people are exacerbated by the abuse of ecosystems and global climate change looks certain to increase the stresses and variability they face. ~~To help shape a research programme proposed by the UK Department for International Development (DFID), this report seeks to highlight some of the critical issues facing water ecosystem services in Africa, South Asia and Latin America and makes recommendations on the research that is needed to fill the current gaps in knowledge and practice.