Leaf temperature increases as stomates close and plant water uptake decreases. The difference between leaf and air temperature can thus be used to characterize plant water stress in response to atmospheric and soil water conditions. In 1981, Idso et al. demonstrated an empirical method for inferring crop water stress from the canopy to air temperature difference, and called it the crop water stress index (CWSI). Later that year, Jackson et al. demonstrated a theoretical method to determine the CWSI based on energy balance principles. The empirical method has been refined over the past 25 years. Recent studies have used linear regression to estimate the effects of solar radiation, air temperature, wind speed, and crop height on the upper and lower baselines used to define the index. The theoretical approach has not been widely used because it requires measurements of net radiation, soil heat flux, aerodynamic resistance, and canopy resistance at full transpiration. We demonstrate CWSI measurements using the theoretical approach with calculated and measured estimates for the necessary parameters. This method has broader applicability over a wide range of climates and crop types. This method also facilitates continuous monitoring of CWSI over the course of a day, and allows the calculation of a value for canopy conductance. Continued improvements of the CWSI facilitate its application for irrigation scheduling, which has been limited because a low-cost, automated system has not been commercially available.