The Bundle of Dreams Protective Fitted Sheet is made of the same Organic Cotton we use on our mattresses. Unlike waterproof mattress covers, ours is 100% breathable. It is water repellent for up to 12 hours and can be machine washed and dried. It can be used under a crib sheet or on its own. Besides for being easy to remove, the elastic perimeter allows for it to fit over any standard 5" or 6" crib mattress.