<prompt> Cilioretinal A is a branch ofAnterior ciliary aeryLong posterior ciliary aeryCerebral retinal AeryChoroidal aery  <response>: The posterior ciliary aery PCA circulation is the main source of blood supply to the optic nerve head ONH, and it also supplies the choroid up to the equator, the retinal pigment epithelium RPE, the outer 130 mm of retina and, when a cilioretinal aery is present, the entire thickness of the retina in that region, and the medial and lateral segments of the ciliary body and iris. That makes the PCA circulation the most impoant pa of the ocular and ONH circulation. Therefore, disturbances in the PCA circulation can result in a variety of ocular and ONH vascular disorders, causing varying degrees of visual loss. I have investigated various aspects of the PCA circulation in health and disease by anatomic, experimental, and clinical studies since 1955. The objective of this lecture is essentially to summarize the main findings of those studies. It is beyond its scope to review the extensive literature on the subject that has accumulated over recent years. 