BACKGROUND: A significant proportion of patients diagnosed under the broad classification of open angle glaucoma actually has normal tension glaucoma (NTG). It has many clinical features that overlap with primary open angle glaucoma (POAG), yet there is a question of whether it has a different aetiology in which intraocular pressure plays less of a role. METHODS: The epidemiology and clinical features of normal tension glaucoma are reviewed with particular reference to possible differences from primary open angle glaucoma, which might permit differentiation. The pathophysiology is discussed, outlining recent research in cell death (apoptosis), axonal damage and neuroprotection. DISCUSSION AND CONCLUSION: There is considerable evidence that NTG develops with little contribution from the effect of intraocular pressure. However, the clinical diagnosis of NTG is often one of exclusion and the differentiation of NTG from POAG remains difficult because many clinical signs are suggestive but not definitive of NTG. More accurate diagnosis may be possible when individual patients exhibit a greater number of signs. Some evidence suggests that NTG with relatively high pressures (greater than 15 mmHg) is more likely to progress than NTG with relatively low pressures. Clinicians must be particularly alert to the possibility of NTG because IOP, a clinical marker for some glaucomas, is absent.