In explaining the damages of primary open-angle glaucoma, the elevated eye pressure was assumed to be the central causative factor. But epidemiological research found high proportions of the following clinical entities: only elevated pressure without further damages (called ocular hypertension), and only glaucomatous damages without high pressure (called normal- or low-pressure glaucoma). These facts made the pressure-theory doubtful, and therefore it had to be modified. Some authors, though, doubt the general validity of the pressure-theory, and postulate other models for the etiology of glaucomatous damages. According to current knowledge, for the diagnosis and screening of primary open angle glaucoma, emphasis should be put rather on the visual fields and the appearance of the optic disc, while pressure measurements supply further clues.