Used from 0F to about -10F on Mt. Washington, NH for 2 seasons. Heavy and bulky, but reasonably warm. I wouldn't recommend them for more than an overnighter due to lack of removeable liners, and leather construction which dries too slowly. Great ball-bearing lace system. Upper toe box needs more insulation. OK for the low cost, but not recommended for deep cold, or long treks. I'd buy a 1/2-size big, and use Sealskin(tm) Neoprene socks, or a vapor barrier and an extra pair of thick socks for extreme cold. Great bomb-proof construction should last forever.