Reliable measurements of the intraocular pressure (IOP) are necessary for glaucoma patients. The measuring procedure, anatomical changes of the eye or extraocular influences can lead to errors during applanation tonometry. The IOP is overestimated if measured through the wrong eyepiece of the slit lamp, if a force is interfering with the tonometer arm during the measurement, if the lid comes into contact with the tonometer tip, if blepharospasm occurs, if there is lid retraction or if a Valsalva maneuver is present. An underestimation of the IOP occurs if staining with fluorescein is absent or insufficient, if the illumination is not bright enough, if a corneal stromal edema is present, after LASIK, during accommodation, during repeated measures within a few minutes or during systemic blood pressure drop. Discrepancies of the IOP in both directions can appear if the calibration of the tonometer is irregular, if abnormal central corneal thickness or astigmatism is present. For correct measurements calibrations are advised once or twice a year.