In order to provide water security in the twenty-first century, there is universal agreement that a continuation of current policies and extrapolation of trends is not an option. Also clear is that from both water supply and development perspectives, the world's arid and semi-arid regions are those currently and potentially experiencing the highest water stresses. One third of the world's land surface is classified as arid or semi-arid, and about half of all countries are directly affected in some way by problems of aridity. The hydrology of arid and semi-arid areas is also known to be substantially different from that in more humid regions. It is therefore essential that investigation methods appropriate to the former are developed and applied, and that strategies for arid and semi-arid region water resources development recognise the principal characteristics of in-situ hydrological processes.