Cystoid macular edema has become the most common and troublesome complication following cataract extraction. New techniques in cataract extraction, such as phacoemulsification and intraocular lens implants have not eliminated the problem, and careful studies are needed to determine whether they exacerbate or diminish it. Cystoid macular edema occurs in many areas of ophthalmology: cataract surgery, retinal surgery, uveitis, ocular tumors, diabetes, and arteriosclerotic vascular disease. Every ophthalmologist must, therefore, be thoroughly acquainted with methods for its recognition and with the inflammatory and vascular factors in its development. Prognosis and treatment are discussed, and the major gaps in our present understanding of pathogenesis are indicated, including the manner by which the vitreous changes following lens extraction lead to inflammation and the reason for the specific susceptibility of the mascular and disc capillaries to the inflammatory stimulus. Some hypotheses are presented.