After reviewing the definition, incidence and main etiologic factors of cystoid macular edema (CME) the authors analyze the structural and physiologic characteristics of the macular area, including the blood-retinal barriers, and the basic concepts of retinal edema. Personal observations on three different groups of diseases associated with CME, retinitis pigmentosa, pars planitis and aphakia, are presented. CME evaluation was made by slit-lamp microscopy, fluorescein angiography and vitreous fluorophotometry. In the situations examined, the main factors involved in CME formation appear to be a marked increase in blood-retinal-barrier barrier permeability and alterations in retinal tissue compliance.