Iridology is the study of the colored part of the eye (called the iris) to determine potential health problems. Iridologists believe that changing patterns and markings in the iris can be used to reveal emerging conditions in every part of the body and to identify inherited weaknesses that may lead to physical and emotional disorders.

Iridology cannot detect a specific disease. Rather, it is a preventive practice that helps people to understand their basic health issues so that they can seek treatment, if necessary, from the appropriate specialist. According to iridologists, if a health problem is detected at an early stage, something can then be done to prevent it from becoming a full-blown disease.

The idea that the eyes are a mirror to the body is an ancient one: The Greek physician Hippocrates was known to examine patients' eyes for signs of illness. It wasn't until 1670, however, that the first actual medical reference to iridology as a diagnostic tool appeared in German physician Phillipus Meyens' book Chiromatica Medica.

In the late-19th century Hungarian physician Ignatz von Peczely and Swedish clergyman Nils Liljequist independently advanced theories connecting the markings of the iris with tendencies toward specific ailments. Both men's interests stemmed from experiences in their childhoods.

Iridology was taught at Harvard Medical School until 1985.