The development of infrared pupillometry in the early twentieth century initiated a rapid increase in studies on the pupil that is continuing today. The chapter will present a brief survey of how the early bulky photographic methods evolved into modern compact instruments.

Interest in the pupil of the eye dates back well over 2,000 years. The prominent philosophers of ancient Greece and Persia had ideas of why the pupil was necessary for vision. Some of the greatest thinkers of the Middle Ages and the Renaissance such as Leonardo da Vinci, Johannes Kepler, Robert Whytt, and Roger Bacon thought about the pupil and presented ideas relating to its function. Slowly but incrementally, our modern ideas of the pupil were formulated by these early pioneers of pupillary physiology.

Traumatic brain injury and ischemic insults to the brain can produce profound changes in pupil size and reactivity. Today, this area of medical care is the most common application of portable infrared pupillometry. An extensive bibliography is included in this chapter that outlines the prognostic value of pupillary measurements after brain injury. Caution is advised. Rare pupillary syndromes such as the tonic pupil can confound these predictions.

The author is an anesthesiologist who has been clinically active for 50 years and has used portable pupillometers since they were introduced in 1989. Ten case reports are presented. Because the author is an anesthesiologist, it is not surprising that the reports in this chapter derive from critical events and drug-induced alteration in pupillary reactions.