Base layer is essentially the "base" of your gear. UA ColdGear wool is the layer to trust. The Coating on the inner layer of your garment absorbs and retains your own body heat so you stay warmer longer. With the right set-up there is no need for bigger bulky jackets that weigh you down. Being a 50% Polyester and a 50% Wool they are extremely comfortable and you hardly can feel them when have them on. Also with their quick drying fabric they feel great for morning hikes or runs where you want to sweat and breath. Under Armour is the ones who started the base layer world and you know they are going to finish. Perform your best with the right clothing.