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The hydrology of desert areas is still poorly known due to the paucity of recorded data and the difficulty of collecting information. The single, overwhelming control over processes is climatic aridity, which involves discontinuous hydrological processes. In deserts, the impact of flowing water on soils depends less on rainfall totals than on lithology and soil characteristics. Most of this water flows in the form of multiple rills or sheet wash, but can also be confined to channels ("wadis"). Intermittence is the rule and occurs as either short-range or long-range floods. A proportion of water inputs is evaporated, another percolates and supplies groundwater reserves. In all the large deserts of the world, underground water is always present and yet of unequal accessibility. The presence of stagnant waters is favoured by tectonic structures and by the specific characteristics of surface processes that are typical of and environments. This is particularly well expressed in the form of elementary water bodies (puddles, ponds), flood outwash basins (playas, sebkhas), and salt- or freshwater lakes with complex hydrological regimes. Old drainage networks, inherited from more humid periods, together with present, intermittent networks, have left uneven marks on the topographic surface. Both an absence of drainage and internal drainage directly evidence the role of climate upon desert hydrography in the recent but also the more distant past.