The amnion is an extremely thin tissue, or membrane. It encloses a developing embryo, and is present within the eggs of all vertebrates which live on land. The presence of the amnion creates a cavity, called the amniotic cavity. The amniotic cavity is filled with amniotic fluid, in which the embryo floats. This fluid serves to cushion and protect the tiny embryo, acting as a kind of shock absorber. The presence of this fluid surrounding the embryo also prevents the thin, delicate tissues ofthe embryo from drying out, at the same time preventing the embryo from sticking to the inside of the egg shell. The amniotic fluid also helps maintain the embryo in an environment with a consistent temperature.

The amnion is sometimes popularly called "the bag of waters." At some time during a human birth, this bag of waters bursts.