The aqueous humor is a liquid substance found in the eye chambers of almost all creatures with the ability of sight. Made mostly of water, the aqueous humor delivers vital nutrients to the eyes, as well as serving a functional purpose in maintaining correct pressure balance in the eye chamber. By filling both the anterior and posterior segments of the front of the eye, the aqueous humor not only ensures that the eye has enough nutrients to work properly, it actually forces the eye to maintain its shape.

The concept of humors has existed in medical research for over 2000 years, when ancient physicians believed that the health of the body depended on the behavior of bile, blood, and phlegm or water. These substances, all fluid in nature, came to be known as humors. Although most theories regarding the importance of the humors have been debunked by modern medicine, the aqueous humor retains the name.

To create the aqueous humor, ciliary tissues surrounding the eye secrete the mostly water-based fluid, which is then transported between the lens and iris of the eye. After passing through the pupil, the fluid then drains out of the eye through a small layer of tissue called the trabecular meshwork, before being reabsorbed into the bloodstream. As it passes through the eye, the aqueous humor nourishes the lens and cornea with glucose and other important substances. The continual movement of the fluid through the front of the eye maintains the pressure needed for an eye to retain its shape.

Correct pressure of the aqueous humor is vital to maintaining eye health. If the pressure level is consistently too high, conditions such as glaucoma can develop in the eyes. Glaucoma is a fairly common condition in which damage to nerves in the eyes eventually can lower vision ability, causing partial or full blindness over time. The disease can occur in one or both eyes and is diagnosed through vision tests that measure inner-eye pressure. People with high blood pressure, of certain racial descents, and the elderly, are more likely to develop glaucoma.

The necessary function of the aqueous humor has lead to numerous studies regarding its effect on other eye diseases and conditions. Studies of agriculture have shown that the aqueous humour of farmed salmon carries less amino acids then that of wild species, possibly contributing to higher level of cataracts in farmed varieties. Because of the fluid's quick and noticeable reaction to any type of drug treatment, researchers often examine the composition of the humor in order to tell of anti-inflammatory treatments are working. In the search for better methods of preventing eye disease and curing blindness, this simple but vital fluid remains a major focus of scientific study in the 21st century.