The eye is largely exposed to foreign substances, and ocular tissues have a complete array of immune cells to interact with offending antigens. In particular, the external eye represents an ideal site for immediate hypersensitivity reactions because of the high number of mast cells in the eyelids and bulbar conjunctiva, as well as for the potential local synthesis of IgE. In fact, the involvement of the eye was part of the original description of hay fever by Charles Blackley in 1873. In a large epidemiological survey of allergic patients we reported since 1987, 40% had symptoms possibly related to allergic conjunctival disease. However, the participation of the conjunctiva to the multiorgan picture of allergy is largely underestimated. Allergic eye diseases have long been referred to the classical Type I hypersensitivity mechanism according to Coombs and Gell. Recently, however, the tremendous progress in understanding allergic mechanisms and inflammation has brought new insights into the pathophysiology of several allergic diseases, including those of the eye. Accordingly, several systematic descriptions of allergic eye diseases should be revisited. In particular, the classification of the various forms of a 'red eye' of allergic origin appears inadequate to answer the progress achieved in their multifactorial pathogenesis.