I agree with another reviewer from Chicago (I am in Wyoming with icy cold winds), these are not the warmest baselayer. However, if you are doing something active, like skiing, snowboarding, or maybe snowshoeing without a lot of down time in between, these are a good moisture-wicking layer. They are definitely lightweight, so just keep that in mind, but sometimes I like to wear my long johns to work under my jeans in the winter even if I'm going to be staying in the office, so these will be good for that and I won't overheat when I go outside. I'd go with something heavier if I'm going to be outside for long periods of time without sustained movement. As far as warmth, I'd compare these to silk, but I personally hate silk long underwear and definitely prefer the smartwool. I got the green ones, and I love the color. Size-wise, I got the XL and I am a 5'8" muscular lady about 180lbs, 40"+ hips, and they fit perfectly without being spandex tight and are long enough.