Our most advanced performance base layer is 3D-knit to provide a tailored fit with mesh ventilation to balance warmth and breathability Fit & Design: Slim fit intended to follow the contours of your body Featuring Intraknit(TM) technology: Insulation, articulation, and ventilation are seamlessly integrated to help with movement and comfort Merino Sport yarn helps combine the moisture management, temperature regulation, and odor resistance of Merino with polyester Seamlessly knit mesh ventilation 3D mapping results in near custom fit and freedom of movement