Disability glare describes the loss of retinal image contrast as a result of intraocular light scatter, or straylight. It has increasingly important modern implications such as with cataract and refractive surgery or high-intensity lighting. However, its measurement has proven difficult despite many varying approaches. This article aims to explain the principles and problems associated with glare testing that are important for assessment and use of glare measures. After defining disability glare, this article discusses the importance of its measurement to current clinical practice. It explains the principles and mechanisms that underlie disability glare and its measurement. Finally, some examples of available glare tests are described to illustrate the principles discussed.