The Smartwool Men's Merino 250 Baselayer Crew Boxed made from pure merino wool is thermal underwear as nature intended. Merino wool not only protects the sheep, but also provides warmth, temperature-regulating properties and excellent moisture management for humans. Added to this are the close-fitting, anatomical cut, the flat seams and the panels on the shoulders to prevent the backpack from rubbing. A light and breathable long-sleeved shirt that is perfect for outdoor activities.