Bought these for my son for his Winter Camping with the Boy Scouts. He said he feet never got cold and the temps were high of 15 degrees during the day and -5 degrees at night.
I frequently have had a problem with ski socks - too expensive, too thin & not warm enough. These 40 Below's make a stunning difference. You'll sweat wearing them inside, but they are the only socks I've found that keep my family comfortable for a day of skiing -- even when the temp. is around 0 degrees F.