Historically, intraocular pressure has been the key classification criterion for glaucoma. However, the boundaries of normal IOP remain a subject of debate despite the abundance of criteria that define the patient's status. A critically important but often unknown parameter is the individual baseline IOP of the patient prior to the onset of the disease. Another important aspect is the role of tono- meters and the potential measurement errors that depend on concomitant local and systemic factors. Which devices provide the most accurate IOP measurements, and how can measurement errors be avoided? In addition, there is a need for a more thorough approach to the interpretation of tonometry results in patients with normal tension glaucoma, where IOP is within the "normal" range but the disease continues to progress. These issues remain relevant and still require further discussion. The modern grading of IOP levels in individuals diagnosed with glaucoma requires a shift from average population norms to more personalized algorithms, which should be considered in future glaucoma classifications, given their significance for diagnostic search and assessment of disease progression risk.