Glaucoma is currently the second most blinding eye disease in the world. The vision loss and visual field defects caused by glaucoma are irreversible. A common feature of glaucoma is pathologically elevated intraocular pressure leading to optic nerve damage and visual field defects. IOP is now widely recognized as a risk factor for glaucoma and is our primary treatment target for glaucoma. So what is IOP? What is the normal range of IOP? IOP is the pressure of the contents of the eye against the wall of the eye. When the IOP exceeds what the optic nerve can tolerate, the enlargement of the optic disc depression and defects in the retinal nerve fiber layer characteristic of glaucoma will occur. The normal IOP range is 10-21 mmHg, with a bilateral IOP difference of less than 5 mmHg and a 24-hour IOP fluctuation of less than 8 mmHg. When any of these indicators is outside the normal range, the risk of glaucoma may be present. But does high IOP necessarily mean glaucoma? Does it mean that glaucoma is not present if the IOP is in the normal range? In our clinic, we have some patients who have repeatedly measured IOP above 21 mmHg, but no damage to the optic nerve or visual field defects have occurred during long-term monitoring, so we do not diagnose them as glaucoma, but as “hypertelorism”. In other patients, although the IOP measurement is always within the normal range, or even the 24-hour IOP monitoring results are within the normal range, but the typical glaucomatous optic nerve damage and visual field defects are present, we also diagnose it as “normal IOP glaucoma”. It is clear that the key to the diagnosis of glaucoma is the presence of characteristic optic nerve damage and visual field defects. Glaucoma optic nerve damage can occur regardless of the IOP measurement, as long as the IOP exceeds the tolerance of the optic nerve. Therefore, the key to the treatment of glaucoma is to lower the IOP to a level that the optic nerve can tolerate, so that the damage to the optic nerve and the visual field defect do not develop further. This IOP is called the “target IOP”. Because the capacity of the optic nerve decreases gradually at each stage of glaucoma, the target IOP will be reduced at different stages.