Publisher Summary A number of methods exist for improving the efficiency of water use. The methods can be classified in three broad categories: increasing the efficiency of water delivery and the timing of water application, increasing the efficiency of water use by the plants, and increasing the drought tolerance of the plants. Water use efficiency (WUE) is defined as “the total dry matter produced by plants per unit of water used.” The fraction of the crop that is economically valuable, termed the “harvest index,” is part of the total dry mass and thus part of WUE. The most accurate means of measuring water use efficiency is to monitor the evapotranspiration and harvest the crop for biomass measurements at the end of the season. With the increased ability to transform plants genetically in recent years, it is desirable to apply the tools of molecular biology to the improvement of WUE. Plants showing improved growth with limited water are considered to tolerate drought regardless of how the improvement occurs or whether the water use efficiency is affected. Of the three forms of drought tolerance, dehydration tolerance is most intriguing because it often requires only slight repartitioning of dry mass. The chapter outlines approaches to improving drought tolerance and presents examples of improved drought tolerance. Reproductive development holds particular interest for improving drought tolerance in crops because a large part of agricultural production is devoted to the reproductive parts of plants. When seeds mature, it is common for them to dehydrate as part of the maturation process. These seeds are exposed somewhat to the atmosphere and are known to desiccate to a large extent by evaporation to the air.