I originally ordered this thermal underwear in a large size (my usual shirt size) but it was too tight. I sent it back and ordered again in size XL which fits me perfectly. Aside from the size issue, the thermals themselves are great. I bought them specifically for running in cold weather and the blend of wool and moisture wicking materials is enough to keep me warm and dry during my runs. Thermal underwear is thinner but should be paired with a light jacket to keep you warm when outdoors in above freezing temperatures.