In 2000 an estimated 66.8 million people worldwide will have glaucoma, 6.7 million of whom will be bilaterally blind from irreversible optic-nerve damage. Yet even in developed countries with public educational programmes that target glaucoma, half of the individuals with glaucoma remain undiagnosed. Patients with even mild visual impairment secondary to glaucoma may have difficulties with mobility, driving, and social interactions. Although glaucoma may be associated with increased eye pressures, its diagnosis does not rely on a specific level of eye pressure. Diagnosis of glaucoma often relies on examination of the optic disc and assessment of the visual field. The two most common types of glaucoma--primary open-angle glaucoma and primary angle-closure glaucoma--have different risk factors. Although similar medications can be used to treat these two types of glaucoma, the overall management of patients differs in important ways. Until recently, there were no randomised clinical trials that showed the effectiveness of lowering eye pressures with medications or surgery in patients with glaucoma. However, in 1998 a randomised clinical trial showed the benefit of lowering eye pressure in patients with glaucoma who had eye pressures of 24 mm Hg or less. Because glaucoma is treatable, and because the visual impairment from glaucoma is irreversible, early detection of the disease is critically important.