This article summarizes present knowledge of the vulnerability to water scarcity of semi‐arid Third World countries. Their predicament is explained in terms of four parallel modes of water scarcity, superimposed on one another, two of natural origin and two human‐induced. The authors conclude that long‐term planning within the environmental constraints imposed by water scarcity is crucial, and calls for a new awareness among high‐level experts and policy makers. Careful land and water use planning based on the water‐balance method is a key component, but depends on expanding traditional water resources assessment methods developed in the temperate zone to incorporate root zone water storage, differences in groundwater recharge, and landscape zonation in water‐producing v water‐consuming or evaporating areas.