Why would you need a rain cover when the Eberlestock backpacks are made from waterproof NT1 fabric? If your going out on an extended trip or if an unforcasted driving rain suprises you, you'll appreciate having a rain cover along. Or maybe you be bivving your pack. Its big enough to go over the Just One backpack and is fastened with and agressive elastic edging that pulls tight around the pack. Made from high quality polyurethane-coated nylon. Better safe than sorry. Coyote Brown. 4oz.