The egg coat is the layer of glycoproteins and other organic material which surrounds an egg (ovum), just outside of the egg's cell membrane. The coat protects the egg, and for organisms which engage in external fertilization (fertilization outside of the parent organisms' bodies) it can help prevent the sperm of other species from entering the egg.

For the eggs of mammals, the egg coat is called the zona pellucida and for sea urchins (a popular study organism for developmental biologists) it is called the vitelline layer.

Some or all of the information in this writeup was taken from the science dictionary at; I oversaw the development of the dictionary (the website was mothballed in 1998) and believe I wrote the entry this writeup is partly or wholly based upon.