The term ‘intraocular pressure’ has traditionally been referred to the ‘pressure in the eye’, although in a strictly physically meaning, the ophthalmologic term ‘intraocular pressure’ only describes the transcorneal pressure difference between the intraocular space and the extraocular surrounding air. An eye with an ‘intraocular pressure’ of 20 mmHg thus has a physically correctly defined pressure of 780 mmHg as sum of the atmospheric pressure (760 mmHg) plus 20 mmHg. For more than 150 years, the intraocular pressure (IOP) defined as transcorneal pressure difference has been considered to be the main risk factor in the pathogenesis of glaucoma. It is, however, the transoptic nerve head pressure difference that directly affects the optic nerve head (Fig. 1).1,2 It is defined pressure in the intraocular compartment minus pressure in the retrobulbar cerebrospinal fluid space (orbital cerebrospinal fluid pressure [CSF-P]). Taking into the orbital CSF-P as counter-pressure against the IOP, hypotheses may be formed on the pathogenesis of optic nerve head diseases such as glaucoma, retinal vein occlusions and arterial hypertensive retinopathy.