This classic Smartwool base layer is a merino wool layer for keeping active in cold weather. It's all about next-to-skin comfort as the Slim Fit runs close and 100%; merino wool fabric regulates body temperature. Designed to keep you toasty warm, it still breathes easy when you turn up the pace and start to sweat. A 1/4 length zip at the front helps adjust that comfort level even faster when you're on the move.