Normal-tension glaucoma, also known as low-tension glaucoma, is defined as glaucomatous damage to the optic nerve and visual fields with normal diurnal values of intraocular pressure (IOP). The term 'low-tension glaucoma' is not often used because in most patients with normal-tension glaucoma, the IOP is within the higher range of normal values and rarely low. The diagnosis is insidious in many cases and requires a complete and thorough work-up to exclude other causes for optic disc and visual field abnormalities. The definition is problematic because the normal limits of IOP have a wide Gaussian curve range and their effect on the development of glaucoma varies. Some patients may retain a high IOP for many years without any glaucomatous damage, while others with low values of IOP may suffer from ongoing progressive glaucomatous disease. IOP is considered as a risk factor for the advancement of glaucoma even in patients with normal values of IOP, and lowering the IOP often protects the optic nerves (Collaborative Normal Tension Glaucoma Study Group [CNTGSG], 1998). Some optic nerves are more vulnerable even to low levels of IOP than others (Drance et al, 1973). Though many factors have been suspected and investigated, it appears that in addition to variability of the structure of the lamina cribrosa, vascular and genetic factors are most likely involved. Most authors consider normal-tension glaucoma to be a variant of primary open angle glaucoma (POAG) (Caprioli & Spaeth, 1984; Chumbley & Brubaker, 1976); others rely on characteristic clinical features of many normal-tension glaucoma patients to consider it a distinct entity (Caprioli & Spaeth, 1984; Shields, 2008). The debate is ongoing and will probably continue to be the subject of research for many years.