Fuzzy (like short fur) polyester. Lightweight and warm for above-freezing (West Coast) weather, like a medium-weight fleece. The small is like a men's small so there's plenty of room on size 4 me to layer flannel shirts and a sweater underneath, and the inevitable waterproof layer on top. I really like it and it's become a go-to, although I hesitated for a while at the price. Washes well.
