I use them for work, I'm a metal worker and sometimes work outside in the bitter cold of winter. They are quite expensive but definitely worth their price in the end. adjustable for all sizes, I have M and L. The material is very stretchy and even if you order a size down it will most likely fit you after a few uses and a good wash in cold water. I've worn it at 10 - 15 °F for 6-8 hours and can say it keeps you warm no matter what, I actually do feel a bit sweaty if I move too much while wearing it under my work clothes !very thin, but like Under Armor always does when needed to generate/retain body heat! This is a very good product made from the highest quality materials (which most UA products are made from) other than that I have a set of UA Hoodies with the same thermal quality and fine material design and thoroughly enjoy all of them! 5 Stars!