Metabolic demands of the ocular tissues are determinants for the presence, morphology, and permeability of the ocular blood vessels. Changes in those metabolic requirements, such as those which occur during tissue maturation or a disease process, can lead to an alteration of the unique permeability and morphology of each nutrient vessel system. Those alterations during the course of a disease are the basis for the well-known visually destructive complications associated with vascular retinopathies. In order to prevent or circumvent those complications, we must come to understand both normal and abnormal tissue environmental factors which are determinants of vessel morphology and permeability. Investigations of those determinants in animals having fully, partially, or non-vascularized retinae and in animals with experimental retinopathies are necessary to the understanding of tissue/blood vessel inter-relationships. It is also necessary to evaluate those same relationships as fully as possible in human eyes with and without vascular abnormalities. Then we can begin to understand and to control blood vessel development.