For more than 50 years Goldmann applanation tonometry has been the internationally accepted method for measuring intraocular pressure. In Goldmann applanation tonometry, however, some basic physical properties are oversimplified and the method has some flaws and restrictions. This paper is intended to promote the understanding of the methodological basis of Goldmann applanation tonometry and describes the most important factors influencing the measurement of intraocular pressure (e. g., corneal thickness, corneal radius, axial length and corneal morphology). Furthermore, the basic principles of other commonly used tonometer devices will be discussed. New developments are anticipated that will measure the true intraocular pressure more accurately than Goldmann applanation tonometry.