This chapter describes the various aspects of crop water deficits. The water status of a crop plant is usually defined in terms of its water content, water potential, or the components of water potential. The simplicity of measuring water content led to its early adoption, but the diurnal and seasonal changes in dry weight make comparisons of water content at different times of day or during the season unsatisfactory. Water deficits develop inevitably as a consequence of water loss from the leaf as the stomata open to allow the uptake of carbon dioxide from the atmosphere for photosynthesis. There have been developments in both direct and indirect methods of measuring water deficits that have resulted in considerable progress in the field. The use of infrared thermometry for the measurement of crop water deficits and the use of in situ psychrometers for the measurement of water potential respectively is elaborated. It is found that the identification of the root as the site of sensing soil water deficits does not eliminate the role of turgor pressure as the transducer of water deficits, but moves the emphasis from leaf to the root. It is observed that at the whole crop level, the water use efficiency will depend not only on the transpiration efficiency of the leaves, but also on the water loss from the soil and the optimization of yield per unit of water used.