The US GI Gore-Tex Modular Sleep System is an extremely versatile and rugged 3-part sleeping bag designed to offer the user up to 4 hours sleep at -40 degrees F in heavy precipitation when used with base layer, hat, and socks. Most users will never need a sleeping bag rated for such extreme conditions, but the MSS remains a highly versatile trio of nesting, attachable/detachable sleeping bags. The outermost layer, and centerpiece of the system, is the water- and windproof gore-tex bivy bag (note: newer versions of the MSS have the ACU camo pattern). It can be used alone or with a light liner in mild weather. The patrol bag (not shown here) is rated to 30 deg F, and the Intermediate Cold Weather bag to -10F. Used together, they will keep you warm down to -30 F, and the addition of the gore-tex bivy completes the 3-layer system for maximum cold protection. Additionally, heavy zippers add to the durability of the system, and all bags have adjustable hoods with nylon cinch cord. Heavy snaps secure each bag within the nesting system. The strength of the system is its versatility and durability, but this comes with a substantial weight penalty. Other limitations are its bulk and poor compressibility. Weights: bivy bag 35.15 oz, ICW bag 67.4 oz, patrol bag approx 55 oz, compression sack 14.05 oz. Total weight: about 11 pounds, but this system also fulfills some of the same functions as a separate tent shelter. I believe consumers will benefit from buying only the gore-tex bivy <b>...</b>