This article is the first of a two-part series on allergic rhinitis. Allergic rhinitis, which includes seasonal allergies, has been considered by physicians, parents, and the general public as a mild, transitory nuisance for children and adults that is easily remedied with over-the-counter or prescription medications. Recent research, however, suggests that allergic rhinitis is more pervasive and deleterious than previously recognized. In addition, medications used to treat allergic rhinitis can have side effects that impact alertness, attention, memory, and physical growth. This article reviews the definition of allergic rhinitis; research on the physiological, educational, and psychological sequelae; and treatments with attention to side effects that affect learning and behavior in schools.