Iridologists use the charts to distinguish between healthy systems and organs in the body and those which are overactive, inflamed, or distressed. Iridologists believe this information may be used to demonstrate a patient's susceptibility towards certain illnesses, to reflect past medical problems, or to predict health problems which may be developing.

As it is not a method of treatment but a diagnostic tool, its practitioners often study other branches of alternative medicine, such as naturopathy.

Iridology is not supported by any published studies and is considered pseudoscience to most medical practitioners.
 

Iridologists generally use equipment such as a flashlight and magnifying glass, cameras or slit-lamp microscopes to examine a patient's irises for tissue changes, as well as features such as specific pigment patterns and irregular stromal architecture. The markings and patterns are compared to an iris chart that correlates zones of the iris with parts of the body. Typical charts divide the iris into approximately 80-90 zones. For example, the zone corresponding to the kidney is in the lower part of the iris, just before 6 o'clock. There are minor variations between charts' associations between body parts and areas of the iris.

According to iridologists, details in the iris reflect changes in the tissues of the corresponding body organs. One well-known practitioner, Dr. Bernard Jensen, puts it this way: "Nerve fibers in the iris respond to changes in body tissues by manifesting a reflex physiology that corresponds to specific tissue changes and locations."^[2] This means that a bodily condition will translate to a noticeable change in the appearance of the iris. For example, acute inflammatory, chronic inflammatory and catarrhal signs may indicate involvement, maintenance, or healing of corresponding distant tissues, respectively. Other features that iridologists look for are contraction rings and Klumpenzellen, which may indicate various other health conditions, as interpreted in context.
 

The first explicit description of iridological principles such as homolaterality (without using the word iridology) are found in Chiromatica Medica, a famous work published in 1665 and reprinted in 1670 and 1691 by Philippus Meyeus (Philip Meyen von Coburg).

This is an example of an iridology chart, correlating areas of the left iris, as seen in the mirror, with portions of the left hand side of the body. Changes in color or appearance of the iris are said to indicate changes in the health of the corresponding section of the body.

This is the corresponding chart for the right iris which relates to the right hand side of the body.