Pars pupillaris of a human iris (near the pupillary border), using false colors to highlight the epithelia (red.)

In anatomy, the iris (classical plural irides) is the most visible part of the eye. It is a circular pigmented tissue containing a stroma, itself containing the sphincter around the pupil of the eye, and two layers of epithelia. In the animal kingdom, only longitudinally symmetric taxa, or chordates, have an iris. This part of the anterior pole of the eye is actively permitting the pupil to open and almost close on a continuous basis. This happens in order for the iris to control the amount of light striking the central retina. In fact, despite pigment content in its stroma and epithelia, the iris is slightly transparent, especially in its peripheral part (pars ciliaris) and at the level of stromal crypts (of Fuchs). Moreover, iris-transmitted light, which is a few orders of magnitude less intense, because of iris stromal absorption, is totally reflected on the anterior surface of the lens, and directed towards the processi dentati of the ora serrata - a region collectively named the "peripheral retina" which processes light only for non-visual purposes.

The iris is the only living tissue always visible naturally, with open eyes.

The iris is brightly pigmented, with colours ranging from red (no pigment at all, like in oculo-cutaneous albinism), to gray, green, blue, basic brown, black, and hazel. There is only one pigment that allows the human iris to absorb almost all the colours of the rainbow, and this is the black pigment called melanin. Structurally, this huge molecule is only slightly different from its equivalent pigment found in skin and hair.

A person's "eye colour" is actually the colour of her or his iris, since the cornea is transparent and the sclera is rarely if ever any other colour than white. Certain eye colours are sometimes seen as being especially attractive and motif-expressing contact lenses can be worn to mask one's natural eye colour with another. They are rarely needed and almost never recommended by serious medical doctors, unless the patient's retina needs extra protection, as in aniridia. Although there has been much fuss about finding the genes for eye colour, there is no simple genetic determinism for such a complex trait, as there is more to iris colour than pigmentation, and overall, there is no simple inheritance and consequently no serious test of paternity based on iris colour.

The occurrence of two irides with different colours is a rare situation which could also be perfectly normal and which is named heterochromia iridis. Circular sectors of strikingly different colours in the same iris happen more often and are sometimes described as heterochromia iridium.

When photographed with a flash, the iris only reacts to protect the retina, and not fast enough to avoid the red eye effect.

Acquiring the iris image, sometimes without the person's approval, and storing it in a database for purposes of identification is a biometric method used for the recognition of human individuals. This is a highly controversial method for many reasons, including that person's rights, and its reliability, reputability, and rate of false positive results. Especially controversial is the fact that the iris stromal pattern changes in time, visibly, although in many subtle ways. Therefore questions can be raised about the viability of the iris as an "immutable bar code".