Iridology studies the eye and the area around the eye, particularly the markings of the iris — the coloured portion of the eye. Every organ is connected to the iris through the nervous system. In the first stages of fetal development, the eye is actually part of the brain. As the embryo forms, the eye slowly separates yet maintains thousands of connections to the nervous system. Other nerves in the body receive impulses from the optic nerve, the optic thalamus and the sympathetic and parasympathetic nervous systems. Iridologists believe that illnesses, including the location of a disease, its history and the possibility for progression, are identified by reading the iris. Iridology is a non-invasive way to study a person's physical and mental health.

Each person's iris has a unique pattern, and iridologists keep detailed records of each one. They carefully examine the integrity of the iris's tissue. Tissue integrity is the structure, colour and density of the iris fibres. By examining the individual fibres within the iris, iridologists discover inherent genetic traits. Close-knit fibres and loose fibres suggest different health outcomes.

Iridology does NOT reveal specific diseases or infections, since many diseases cause similar changes in body tissue. Instead, according to iridologists, the iris reflects the condition of these tissues and pinpoints areas of toxicity, congestion or other types of imbalance. Modern iridologists often use a chart outlining the relationships between portions of the eye and various body systems.

Identifying the body's inherent strengths and weaknesses allows a person to do what's necessary for the weaker areas before the symptoms of disease cause further destruction. This may be useful for HIV-positive people whose weakened immune system enables illness to manifest quickly. Ayurvedic treatments are often used in conjunction with iridology to maintain and restore weaker areas of the body.

The effectiveness of iridology is a subject of controversy.  A recent review of the medical literature found that controlled trials to date have not demonstrated any clinical benefit to iridology. The cost of an initial session can range from $60 to $150. Iridologists are not regulated anywhere in Canada.