Glaucoma is a heterogeneous group of optic neuropathies that have in common an accelerated degeneration of retinal ganglion cells and their axons, a subsequent typical excavation of the optic disc and a concomitant pattern of irreversible visual field loss. Glaucoma affects approximately 2% of individuals of European descent and up to 10% of individuals of sub-Saharan African descent over 50 years of age. It is a progressive disease, which without adequate treatment can result in severe visual disability and eventually blindness. Primary open-angle glaucoma (POAG) is the predominant form of glaucoma in Western countries. The disease is distinct from other forms of glaucoma through its age-related, insidious onset and an unobstructed iridocorneal angle with a normal appearance. Traditionally, an elevated intraocular pressure (IOP) was part of the clinical definition. However, an estimated 20 - 50% of all patients with otherwise characteristic POAG have IOPs consistently within the normal range (a condition referred to as “normal tension glaucoma”), whereas most individuals with an elevated IOP do not have any signs of glaucomatous optic neuropathy or visual field loss (a condition called “ocular hypertension”). Nevertheless, an elevated IOP is considered an important causative factor and the major risk factor for POAG. The 10-year incidence of glaucomatous visual field loss has been reported to increase by 11% [6-15%] per millimeter of mercury increase in IOP. Moreover, IOP is currently the only modifiable risk factor. Lowering the IOP, either by medication or surgically, has been shown to reduce the risk of conversion from ocular hypertension to glaucoma and to slow down the progression of glaucoma.