Covers come in many styles and shapes but their function is to provide a waterproof outer layer for non-waterproof diapers (all except AIOs and Pockets). Diaper covers are usually made from polyester, Polyurethane laminates (PUL), waterproof nylon, fleece or wool. This is where you can get creative! There are so many different designs and colors (and you can even get some personalized!) You do not need to wash the diaper cover after every use, they can be reused until they get dirty or retain an odor.