I purchased these booties for high altitude mountaineering and winter camping. With a heavy sock, I would rate them at about 10 deg.~F at best. Nice to have in the tent. They kept my feet dry even in the snow however, I wouldn't recommend them in a foot of snow as they suggest in the North Face description of the bootie. I wear a size 12 and bought the XL. The fit was great with enough room for heavy socks but not too slopy with bare feet. We just returned from several climbs in South America. The highest (near 23k ft) being Cero Aconcagua. I would have liked something a little warmer but the wieght to warmth ratio was not bad.