The vitreous humor comprises a large portion of the eyeball. It is a clear gel-like substance that occupies the space behind the lens and in front of the retina at the back of the eye.

Because the eye must process visual data, this liquid must be clear enough for light to easily pass through it. Most of this humor consists of water, as well as a lower amount of collagen, salt, and sugar.

This humor is a stagnant (immobile) fluid that is not served by any blood vessels and is not actively regenerated or replenished. (This is in contrast to the aqueous humor, which fills the anterior chamber in front of the lens.)

If a substance enters the vitreous humor, it will remain suspended in the gel until it can be surgically removed. These substances, which can include blood or clumps of cells, are collectively referred to as floaters. If left alone, floaters can affect a person's field of vision. As people age, vitreous thins. This can result in a condition called posterior vitreous detachment, where the vitreous separates from the retina. Posterior vitreous detachment occurs in most people by age 70. It can cause floaters but generally resolves on its own over time.

Problems with the vitreous humor may ultimately lead to detachment of the retina from the back wall of the eye, which may require surgery. Retinal detachment can result in permanent loss of vision.