This is one of my two absolute favorite running shirts (and they're almost identical). The M-Sprint is made from the best running shirt material, in my opinion, Nuyarn: all the benefits of merino wool, with added durability and quicker drying properties provided by nylon. This shirt is the perfect weight for running, even on hot summer days. I should say that I only run in a dry climate, so I can't speak to how well this would work in high humidity. But here in Colorado, it's the perfect running weight and material. I use these shirts for trail running, road running, scrambling, and climbing. I've had a few of these shirts for about a year now, with no signs of breakdown. Usually with a pure merino wool shirt, I would start to see small holes by now, caused by abasion, but I've found Nuyarn is much more durable. I've run for years in the BD Rhythm shirt, which uses the same material, and I find them to be very similar (perfect) shirts.