This is part of the ECWCS (Extreme Cold Weather Clothing System). The heart of the Cold Weather Clothing System is the expedition weight polypropylene thermal underwear. Polypro underwear is military tested and issued. The 100% polypropylene material will wick the moisture away from the skin - you stay dry and cozy in bad conditions. The Polypro long underwear have a silky smooth comfort. The long underwear is made of 100% texturized filament yarns, brushed to a thick fleecy nap. The polypro bottoms of the ECWCS Polypropylene Thermals have an elastic waistband, fly flap and knit cuffs. The brown poly pro is machine washable. The thermals are available in sizes Small - XXXLarge.