Building upon the United Nations Framework on Climate Change's (UNFCCC) global, top-down analysis of the costs of climate change, the United Nations Development Programme (UNDP) commissioned this guidebook to support developing countries to undertake a bottom-up, national sectoral analyses of the costs of adapting to the impacts of climate change and mitigating greenhouse gas (GHG) emissions. This guidebook, which was developed by UNDP with a group of international experts and regional centres of excellence, comprises (a) a work plan guidance for preparing for an Investment and Financial Flows (I&FF) assessment; (b) a methodology guidebook for the assessment of I&FF; and (c) reporting guidelines. This chapter is devoted to adaptation in the water management sector.