From Halfdays: Our merino base layer top, a midweight blend of merino wool and bamboo to keep you warm from basecamp to backcountry. - Moisture-wicking technology is fast-drying to keep you cool and comfortable - Merino-bamboo blend is naturally insulating and thermoregulating for optimal warmth - Naturally odor-resistant - Fabric : 51% merino wool, 40% bamboo viscose, 9% polyamide