There are many ways to measure the intraocular pressure (IOP) and all are influenced to some degree by the individual characteristics of the eye. Ocular rigidity (OR) is a familiar concept used in ophthalmology to describe the relationship between pressure and volume changes in the eyeball. This concept partly explains why IOP measurements vary between different types of tonometers. Some tonometers are more heavily influenced by OR than others, due to different amounts of corneal deformation, and volume of fluid displaced during tonometry. More recent tonometers either produce less corneal deformation and less fluid displacement or, attempt to compensate for variations in the biomechanical characteristics of the cornea. Surgical intervention can influence the true IOP but, additionally modify the OR and therefore differentially influence the results from different tonometers. Clinicians should be aware of these effects when interpreting IOP measurements, especially in eyes that have had surgery or have other characteristics that may affect OR. A deeper understanding of the factors that influence OR might lead to the development ofmore accurate instrumentation that is less influenced by OR.