A prime concern in cultivating crops has always been water availability. The earliest crops may have been seeded about 18,000 years ago on the high dunes area of the Nile floodplain immediately after the flood waters receded (Wendorf et al. 1982). This practice assured adequate moisture for plants to grow and produce grain. Plant water-use efficiency was a topic for early scientific study (Briggs and Shantz 1913, Lawes 1850, Woodward 1699). Knowledge of the factors influencing crop water-use efficiency and a hope to improve the efficiency has continued to be an objective in many modern investigations. Wittwer (1975) identified water as the second-most limiting factor, behind land area, to increasing food production. He argued that a high research priority should be an improvement in the efficiency of water use by plants. Considerable research has been done on crop water-use efficiency during the past century, but much work resulted in empirical conclusions that seemed confusing or contradictory. However, recent developments in the understanding of the physical and physiological processes regulating crop growth and water loss allow crop water-use efficiency to be analyzed in quantitative, mechanistic terms.