Additional understanding of stomatal behavior comes from transport of water through the soil-plant-atmosphere continuum based on the principle that plants reduce stomatal conductance as needed to regulate transpiration and prevent hydraulic failure. As xylem water potential decreases, the supply of water to foliage declines and leaves may become desiccated in the absence of stomatal control. Stomata close as needed to prevent desiccation within the constraints imposed by soil water availability and plant hydraulic architecture. This chapter develops the physiological theory and mathematical equations to model plant water relations.