I gave these as a gift to my Grandma who has circulation issues - her hands/feet are ALWAYS cold, even when everyone else is warm/comfortable.  I didn't wear them myself (obviously), but I did flip them inside out just to check out the insulation inside.  If I could describe it, it's like insulation you see in a home but for feet, if that makes sense.  It looks like fuzzy carpet/insulation on the inside with a knitted outside that is SUPER thick.  Great product if you are really trying to stay warm.  Only tip, they are really thick so unless you buy shoes that are 1/2 or a size too big, you might not be able to wear shoes over them.  Lucky for my Grandma, she uses these at home, mostly at night to go to bed.