Mast cells have long been recognized for their role in immediate hypersensitivity reactions, by virtue of the presence of high affinity receptors for IgE (FcεRI) on their surface. More recently, mast cells have been postulated to be involved in a variety of chronic inflammatory disorders as numerous mediators released by activated mast cells are characterized. This article summarizes current information on mast cell mediators, heterogeneity, and differentiation, and reviews studies of mast cells in the normal eye as well as in various ocular disorders.