Over the last decade, evidence has accumulated to support the hypothesis that significant lowering of intraocular pressure (IOP) leads to a reduced progression of glaucomatous optic atrophy and visual field loss. This has been found to be true for patients with established visual field loss, with IOP above the normal range, as in the Advanced Glaucoma Intervention Study (AGIS Investigators 2000) and also in patients with established visual field loss but with normal presenting IOP, as in the Collaborative Normal-Tension Glaucoma Study Group (1998). In AGIS there was an inverse relationship between the level of IOP and the rate of field progression: eyes that maintained a mean IOP of 12mmHg showed no tendency to progress over the 8 years of the study. In addition, in the Ocular Hypertension Treatment Study (Kass et al. 2002) the chance of developing glaucomatous change was reduced by half when IOP was lowered by 20–25%.