Vitreous humour is the clear substance that fills the part of the eyeball lying between the lens and the retina. It is 99.98% water, and the water and salts within the vitreous are in constant exchange with the aqueous. The vitreous humour contains microscopic fibres embedded within a gel. Because of this microstructure, it is only possible to remove the vitreous by cutting it up into small pieces before sucking it out of the eye.

The collagen fibres of the vitreous are held apart by electrical charge. With ageing these charges tend to reduce, and the fibres may clump together. Similarly the gel may liquify (syneresis), leading to vacuoles within the vitreous. These commonly lead to floaters. Floaters are small, generally harmless, defects in the vitreous humour. The sudden onset of floaters may signify a posterior vitreous detachment, PVD, or other disease and should be investigated immediately by a healthcare professional.