The first time I wore this jacket I was running on a snow covered trail in Chicago. I only had a turtleneck base-layer underneath in 27* wheather. I was out for 2.5 hours and warm as could be. I could have stayed outside for another 3 hours! Oh, for reference I am 5'6" 148 lbs, athletic built, DD top and the large fits great, with room to layer. In fact, I love this jacket so much I ordered another one and the hoody vest.