Mooren’s ulcer is an idiopathic, destructive corneal lesion developing in the corneal periphery, typically progressive centrally, centrifugally, and posteriorly, sometimes progressing to full corneal thickness and perforation. First described by Bowen and later by McKenzie, it bears the name of the German ophthalmologist who most fully described it and presented a collection of several cases. Its most unique clinical characteristics include the “eating away” of cornea central to the most obvious crescent of epithelial defect and stromal melting, likened to the gnawing away of tissue that perhaps one could image having been accomplished by a rodent (hence the name in some circles as corneal ulcer rodens); it is not associated with necrotizing scleritis, and many of the collagen vascular diseases. Patients affected with Mooren’s ulcer commonly complain of intense pain, often out of proportion to the degree of clinically obvious ocular inflammation. The disease has the characteristics of an autoimmune process, and both we and others have documented autoimmune phenomena, both in the eye and systemically, in patients suffering from Mooren’s ulcer. It typically responds to aggressive steroid and/or immunosuppressive chemotherapy. Wood and Kaufman described two forms of the disease: one more benign, unilateral form which often responds to intensive topical steroid therapy and/or conjunctival resection, and another more “malignant” bilateral form which progresses despite all attempted local treatments, and responds only to systemic immunosuppresssive chemotherapy. A variety of provocateurs of the onset of Mooren’s ulcer have been described, including trauma and infection (herpes simplex, hepatitis C, parasites, salmonella). Perhaps the more important facts to remember about this disease are that corneal lesions similar to it may be the presenting manifestation of a potentially lethal systemic disease, and therefore careful attention to the systemic evaluation of a patient with Mooren’s corneal ulcer is important; the process can be caused by infection, and curing of the infection can result in resolution of the problem; and, finally, that the bilateral progressive form of the disease usually responds to nothing short of systemic immunosuppressive chemotherapy.