Almost two-thirds of patients with giant cell arteritis (GCA) develop ocular symptoms and up to 30% suffer permanent visual loss. We review the three most common mechanisms for visual loss in GCA, describing the relevant ophthalmic arterial anatomy and emphasising how ophthalmoscopy holds the key to a rapid diagnosis. The short posterior ciliary arteries supply the optic nerve head, while the central retinal artery and its branches supply the inner retina. GCA has a predilection to affect branches of posterior ciliary arteries. The most common mechanism of visual loss in GCA is anterior arteritic optic neuropathy due to vasculitic involvement of short posterior ciliary arteries. The second most common cause of visual loss in GCA is central retinal artery occlusion. When a patient aged over 50 years has both anterior ischaemic optic neuropathy and a central retinal artery occlusion, the diagnosis is GCA until proven otherwise, and they should start treatment without delay. The least common culprit is posterior ischaemic optic neuropathy, resulting from vasculitic involvement of the ophthalmic artery and its pial branches. Here, the ophthalmoscopy is normal acutely, but MR imaging of the orbits usually shows restricted diffusion in the optic nerve.