If you are doing any hard work in a cold environment, cotton is bad. Cotton does not wick so sweat and moisture will accumulate near the body and when you stop moving it will help move body heat away from you. Do not wear cotton next to your skin when climbing/hiking in the mountains - silk and/or polypro and wool wick moisture away from the body. The best underwear I have found is sold by Duluth Trading company - they call it their 'buck naked underwear' they are made of spandex and nylon and yes it does feel like I am wearing nothing. I walk a lot and wearing the normal cotton whitey-tighties would get all sweaty and be a breeding ground for crotch itch.