Recent work has highlighted the eosinophil's role as an effector cell in a wide array of disease entities, including parasitic infections and allergic and nonallergic diseases. The eosinophil is filled with granules containing toxic cationic proteins, capable of harming tissue when released to the extracellular space. In the eye, toxic eosinophil cationic granule proteins have been encountered in conjunctiva, cornea, tears, and contact lenses of patients suffering from ocular allergy, suggesting an effector role for the eosinophil in the ophthalmic manifestations of atopy. Laboratory investigations indicate that eosinophil granule major basic protein, the principal eosinophil granule protein, may adversely influence corneal epithelium, and promote corneal ulceration in the severest forms of ocular allergy. Further, the eosinophil may play a contributory pathophysiologic role in some nonallergic ophthalmic diseases such as Wegener's granulomatosis, orbital pseudo-tumor, and histiocytosis X. The eosinophil's morphologic, immunologic, and biochemical characteristics will be reviewed and its role in certain ophthalmic diseases thoroughly examined.