Wearing one base layer day after day on a multi-day trip puts the fabric to work, and the Olympus Merino Baselayer Long-Sleeve Top answers with 250gsm merino wool that regulates temperature, resists odor, and dries fast. The slim fit stays close to the body under outer layers. What Our Gearhead(R) Experts Are Saying:"On long uphills with a pack on, the thumb loops kept my sleeves in place under my jacket as my arms swung. The flatlocked seams stayed smooth beneath the pack straps."