The use of infrared thermography for the study of plant water relations is reviewed. Increases in leaf temperature detected by thermography largely reflect stomatal closure as a measure of “stress.” Increases in leaf temperature can therefore be used as an indicator for irrigation scheduling. Most other uses of thermography in plant and crop science, for example as a diagnostic tool for a number of plant diseases and environmental pollutants, also depend on the indirect effects of these stressors on stomatal aperture and transpiration rate. Advances in digital image analysis are providing new ways automating image collection and analysis of specific areas within images and for large numbers of images. In spite of the power of infrared thermography, its value for diagnosis and quantification of plant stress will be greatly aided by its use in combination with other sensing techniques such as spectral reflectance or fluorescence imaging.