I say "great base layer"because they are exactly that. They are fairly light overall, not to be confused with expedition-weight underwear. That said, the reflective treatment on the inside is amazing! Use these in conjunction with any other long underwear and you're good to go deep into the cold. The moisture wicking is also very nice. Personally, I use these under my leathers for cold-weather motorcycling. The skin-tight fit is very helpful, since room is at a premium. All in all, I can highly recommend these tights for cold-weather use.