Glaucoma, one of the major causes of blindness in the world, is a progressive optic neuropathy. Elevated intraocular pressure is a well-known major risk factor for glaucoma. In addition, there is growing evidence that vascular factors may play a role in glaucoma pathogenesis. Systemic (e.g. hypertension, diabetes) and ocular vascular factors (e.g. ocular blood flow, ocular perfusion pressure) have been assessed for associations with glaucoma. However, direct and convincing evidence for primary mechanisms of glaucoma is still lacking. The aim of this review is to summarize the evidence implicating vascular factors in the pathogenesis of glaucoma, with particular emphasis on the role of ocular blood flow and ocular circulation as risk factors for primary open angle glaucoma.