In the last 150 years the area under irrigation has increased dramatically, particularly in the drylands of the developing world, having a relatively scarce but renewable water supply. The latter applies to two broad physiographic settings: continental deserts and arid zones in the vicinity of high mountain ranges, where snow-melt provides for large flows in summer, and arid zones where water supply originates in areas of higher rainfall and is conveyed by rivers over large distances. In these regions irrigation has been practised for centuries, but on a significantly smaller scale and the relatively recent expansion, obtained at the cost of huge investments, has brought major improvements in the population welfare as well as environmental degradation through salinization, waterlogging and overdraft of groundwater reservoirs. To some extent the agricultural production system (irrigated agriculture) has developed a significant resilience to climate variability. The rise of intersectoral water competition together with the higher demand for agricultural water use, generated by increasing trade flows, contribute, however, to increase the vulnerability of this production system. For internal drainage basins these trends may imply the disappearance of the remnants of ecosystems which formerly were receiving river water. Possible climate change, especially lower precipitation in arid and semi-arid zones, add a further vulnerability factor to these systems