Ocular allergic diseases are characterized by specific activation of conjunctival mast cells with subsequent release of preformed and newly formed mediators. Mast-cell numbers on the ocular surface are increased in all forms of allergic conjunctivitis. Mast-cell activation plays a central role in the development of the ocular allergic reaction, which can be divided into an early and a late inflammatory phase. Mastcell mediators have been measured in tears of patients suffering from various forms of allergic conjunctivitis, and in sensitized patients after specific ocular allergen challenge. Histamine and tryptase are the most studied mediators in tears of allergic patients. Several cytokines, such as IL-4 and TNFα are also produced and released by conjunctival mast cells, and probably play a role in the immunoregulation on the ocular surface. In vitro studies of the characteristics and biologic functions of conjunctival mast cells highlight their central role in the pathogenesis of ocular allergy, and have led to new opportunities to evaluate anti-allergic compounds. This review discusses the role of conjunctival mast cells in the development of ocular allergic diseases.