I have a bunch of athletic shirts from Patagonia and this is also of a similar high quality/performance. I typically wear a medium when it comes to my workout gear, whereas some of my other Patagonia gear is small (e.g. Down Sweater and Better Sweater). I'm 5ft 7in (with 31 waist) and could go either way in size for Patagonia small/medium, but probably favor small these days. Just depends on cut and what I'm doing. Overall happy with shirt for climbing, running.