Only a few conclusions can be mentioned: The IO pressure of the normal eye is not fixed, but shows spontaneous variations of pulsatory, respiratory, vasomotoric and diurnal type. The highest and lowest point of a 1 minute tracing can differ for 8 mm Hg. Very short acting tonometers are, therefore, less reliable than those with slower reactions, which will average some of the quicker oscillations. Normal limits of diurnal variations are 5 mm Hg with peaks below 25 mm Hg and differences between right and left eye of less than 4 mm Hg. The range of pressure of an eye is more important than a definite value which is true for the moment only. Factors which influence the IO pressure in healthy subjects are discussed in detail, such as age, sex, eye muscles, sports, muscular spasmus, child birth, position of the body, real or simulated tonometry, and mental stress. Difficulties in tonometry are discussed. In applananation tonometry, +/- 2.5 mm Hg must be accepted as the possible error of clinical results. This is not caused by any lack in the construction of the instrument, but by the combination of interobserver variation in reading the instrument, spontaneous pressure changea and an individually different decrease of pressure in consequence of repeated measurements. It is concluded that the rigidity of the living eye cannot be measured with a satisfactory degree of exactness. For the clinical purpose of early diagnosis, the diurnal tension curve combined with the tonography test of the author are the best methods. Early normalization of intraocular pressure is the safest means to prevent damage of the optic nerve. In spite of the individual variations of pressure and the inaccuracies of any single tonometry, the present methods of tonometry are clinically sufficient to form an opinion about the diagnosis and also about the success of our treatment, the aim of which is to normalize the pressure through the 24 hours below 20 mm Hg.