The pathophysiological role of eosinophils and their pivotal position in the network of inflammatory cells involved in allergic inflammation of the eye are outlined on the basis of several in vitro and in vivo personal studies. These indicate that: (1) Conjunctival scrapings and tears from patients with allergic conjunctivitis show a significant increase of eosinophils, their number being related to the activity and severity of the disease. This eosinophilic inflammation is associated with a nonspecific conjunctival hyperreactivity to histamine topical challenge. (2) Eosinophils in inflammed conjunctival tissues of allergic patients are activated, as shown by their immunostaining with antibodies to the secreted form of eosinophil cationic protein (ECP) as well as by ECP presence in the tear fluid. (3) In Sprague-Dawley rats sensitized with DNP-Ascaris, eosinophils recruitment at conjunctival level and persistent eosinophilic inflammation is induced by DNP bis-lysine topical challenge. (4) In humans, an IgE-mediated release of eosinophil peroxidase (EPO) can be induced in vitro by incubation of peripheral blood eosinophils from allergic patients with specific allergen. The new concept of ‘eosinophil releasability’ is introduced to describe the peculiar features of this activation-degranulation process. (5) In vivo, allergen provocation with specific allergen causes in patients with allergic conjunctivitis a significant recruitment and mediator release of eosinophils during the late-phase allergic reaction.