For more than a decade, the Harrier Long Sleeve has been a cornerstone for runners who don't believe in quitting just because the temps dip. Powered by a Merino wool blend that naturally regulates temperature and resists odor, it’s been refined over the years but remains the go-to layer in Tracksmith’s lineup. This latest edition features a heavier knit with 37.5® Technology for advanced thermoregulation, plus a relaxed fit and thumbholes to keep you comfortable from first stride to cooldown.