Treadmills and water treadmills are found in research centers, therapy centers, and are becoming increasingly common in private competition yards, yet little evidence exists which informs their use for rehabilitation of injury. The control that they afford in terms of speed, intensity, and duration of exercise is attractive, but guidance regarding any possible benefits and/or contraindications for treadmill exercise in any given scenario is limited. In this review, the evidence pertaining to the physiology and biomechanics of treadmill exercise in horses is examined and combined with our experiences of using treadmills for rehabilitation over 15 years to offer some basic guidelines as to their use. Evidence is presented to support the use of a land treadmill in the rehabilitation of horses following various distal limb conditions and back pain. The effects of water treadmill exercise on limb and back kinematics are considered and suggestions made as to how to select the most appropriate water depth for various conditions. Successful rehabilitation depends as much on avoidance of unsuitable exercise as selection of beneficial exercise. In time, more evidence regarding the use of treadmills for specific conditions will accrue; but as horses commonly suffer from multiple conditions (e.g., hindlimb lameness and back pain), it is likely that a rationale devised on a case by case basis will always be necessary, with regular monitoring of the gait pattern throughout rehabilitation.