The atmospheric evaporative demand of land surfaces exceeds the rate of soil water supply to vegetation. Root hair for intake and stomata for loss regulate the water deficit of plants. The purpose of irrigation is to overcome the resulting constraint on agricultural productivity. Scarcity of water resources impelled replacing empirical irrigation practices with procedures based on scientific knowledge. Study of water uptake by the root system, flow of water from the soil, through the plant to the stomata and transpiration has revealed the main anatomical, physical and biochemical factors intervening in the operation of the plant hydraulic system. Although water transport mechanisms remain only partially understood, progressive knowledge has resulted in measurable improvements of irrigation practices. Increased water use efficiency entails also better control over leaching and run-off germane to the safeguard of soil and water table.