The concept of a blood~retinal barrier is still relatively new in the ophthalmic literature. ~fuereas work on the blood-brain barrier was initiated in the first decade of this century, the blood- retinal barrier has only recently been defined. Information accu- mulated during the last 10 years has shown that the function of the blood-ocular barriers may be better understood if two main barrier systems are considered to exist in the eye. The blood-aqueous barrier regulates the exchanges between the blood and the intraocu- lar fluids, and the blood-retinal barrier separates the neural tissue from the blood. Recent studies have shown that the blood-retinal barrier plays a fundamental role in controlling the microenvironment of the retina. Similarly, the significance of the blood-retinal barrier in retinal disease has become increasingly clear. Fluorescein angiography has demonstrated an intricate series of relationships between altera- tions of the blood-retinal barrier and diverse retinal diseases, particularly vascular retipathies and pigment epitheliopathies. Finally, in the past few years, vitreous fluorophotometry has pro- vided a new and accurate index of the alteration of the blood-retinal barrier.