Population growth and climate change increasingly challenge water supply systems, and population dynamics, growing demands, pollution, land use changes and other factors frequently produce the failure of some water supply systems. Climate change is believed to augment the existing pressure on water resources. This chapter presents an introduction to the effects of climate change on water resources and analyses their impacts on agriculture. The chapter focuses on several major challenges: the estimation of climate change impacts on rainfed agriculture and water resources, the analysis of current and future water availability, guidelines for climate change adaptation policies in agriculture and water resources and the selection of climate change adaptation options. The chapter also includes a detailed case study analysing water availability for several Southern European basins. The chapter looks ahead to future research topics and provides some suggestions for further reading.