Root water uptake is governed by physiological and physical processes.  Current physiological work focuses on aquaporins, which are a class of water channel proteins that have been found in nearly all living organisms. They are highly expressed in plant membranes.  Their presence in root membranes is of particular significance, because they are thought to enhance the conductivity of the membrane and facilitate water uptake, which is especially important in drought-stressed tissues to promote rapid recovery of turgor upon watering.  Although water uptake by plants is under physiological control, it often is described as a purely physical process, as a consequence of gradients in water potential in the soil-plant-atmosphere continuum (SPAC).  That is, water moves passively through the roots in response to a water potential gradient set up by transpiration.  Models of root water uptake depend upon defining the potentials in the SPAC.  In these models, water uptake is described either by the microscale or macroscale physics of water flow from the soil to, and through, the plants roots.  Both types of models depend upon the water potential gradients.  Future research needs to determine the following:  a way to measure the matric potential at the soil-root boundary; the importance of reverse flow (water movement out of the roots into soil); the magnitude of nocturnal root water uptake; site-specific water uptake by roots; and root resistance.