Ocular allergy is a common hypersensitivity disorder that affects 15%-20% of the population in developed nations. This disorder can be divided into several categories: seasonal and perennial allergic conjunctivitis, atopic keratoconjunctivitis, vernal keratoconjunctivitis, and drug-induced allergic conjunctivitis. Giant papillary conjunctivitis often is included among these classifications of allergy; however, it is not a true allergy, but rather the result of a chronic mechanical irritation. This article reviews these subtypes of ocular allergy, with particular emphasis on the pathophysiology, diagnosis, and treatment of seasonal and perennial allergic conjunctivitis. These are the most common varieties, representing more than 95% of the ocular allergy cases in the United States.