In vivo studies on choroidal vascular bed have shown that the posterior ciliary arteries (PCAs) right down to their terminal arterioles in the choroid have strictly segmental blood flow and they behave as end-arteries. These studies totally contradict the picture shown by all the postmortem cast studies of the choroidal vascular bed as a freely communicating system. The in vivo pattern helps to explain the typically localised nature of inflammatory, ischaemic, degenerative and metastatic nature of the choroidal lesions. Like other end-arterial vascular systems in the body, the choroidal vascular bed has watershed zones situated between the various PCAs, the short PCAs, the choroidal arteries, the arterioles, and the vortex veins. The end-arterial nature of the choroidal vasculature and the existence of watershed zones in the choroid are of great clinical importance; their role in the production of various ischaemic lesions in the choroid, anterior ischaemic optic neuropathy (AION) and other ischaemic disorders of the optic nerve head is discussed.