Allergic conjunctivitis is a response to environmental allergens, as well as a genetic predisposition of the patient. It is classified as either acute (seasonal allergic conjunctivitis) or chronic (perennial allergic conjunctivitis, vernal keratoconjunctivitis, atopic keratoconjunctivitis and giant papillary conjunctivitis). The immune mechanism of these diseases will be discussed, as well as the allergic response to contact lens wear.