This chapter introduces the main components of the global water cycle, describes the main stores and fluxes, and residence times, and presents the underlying physical processes. Uncertainties in the water cycle are large and this chapter discusses those uncertainties. The chapter covers the impacts of climate variability and extremes on the water cycle over different temporal and spatial scales. The impacts of human modification of the water cycle are analysed within this chapter and in particular the role of land-use change and modification of atmospheric composition and climate as a result of anthropogenic emissions of greenhouse gases and aerosols are investigated. The modifications to the water cycle have already been large and are set to be even greater through further human modification. There are therefore major challenges ahead in tackling the changing water cycle under global environmental change and this chapter presents a summary of those challenges.