Oxidative stress refers to the cellular injury and pathologic change that occurs when there is an imbalance favoring oxidants over antioxidants within a living organism. In human medicine, oxidative stress has been implicated in numerous disease processes, which has led to further research into the clinical benefits and efficacy of antioxidant therapy. The evaluation of oxidative stress in the horse has been limited primarily to ischemia-reperfusion injury of the gastrointestinal tract, recurrent airway obstruction, exercise, osteoarthritis, equine motor neuron disease, and pituitary pars intermedia dysfunction. Each of these is examined in this review in terms of the current evidence for oxidative stress as well as the evidence for current antioxidant therapy in equine medicine and the potential of future research and therapies. Oxidative stress research is currently an emerging field with relevance to the equine critical patient.