Horses develop many skin and respiratory disorders that have been attributed to allergy. These disorders include pruritic skin diseases, recurrent urticaria, allergic rhinoconjunctivitis, and reactive airway disease. Allergen-specific IgE has been detected in these horses, and allergen-specific immunotherapy is used to ameliorate clinical signs. The best understood atopic disease in horses is insect hypersensitivity, but the goal of effective treatment with allergen-specific immunotherapy remains elusive. In this review, updates in pathogenesis of allergic states and a brief mention of the new data on what is known in humans and dogs and how that relates to equine allergic disorders are discussed.