Allergic conjunctivitis is in actuality a group of diseases affecting the ocular surface and is usually associated with type 1 hypersensitivity reactions. Two acute disorders, seasonal allergic conjunctivitis and perennial allergic conjunctivitis, exist, as do 3 chronic diseases, vernal keratoconjunctivitis, atopic keratoconjunctivitis, and giant papillary conjunctivitis. The ocular surface inflammation (usually mast cell driven) results in itching, tearing, lid and conjunctival edema–redness, and photophobia during the acute phase and can lead to a classic late-phase response (with associated eosinophilia and neutrophilia) in a subset of individuals. As is the case in other allergic diseases, a chronic disease can also develop, accompanied by remodeling of the ocular surface tissues. In severe cases the patient experiences extreme discomfort and sustains damage to the ocular surface. For such cases, there is no highly effective and safe treatment regimen. Topical administration of corticosteroids is used in severe cases but is associated with an increased risk for the development of cataracts and glaucoma. Thus there is a worldwide search for new biotargets for the treatment of these diseases. Here we provide a brief update of the clinical symptoms associated with these diseases, the rationale for disease classification, recent advances in our understanding of the pathogenesis of the diseases, and an update on both preclinical and clinical advances toward refined therapies for these diseases.