Just bought this shirt a few weeks ago and already I can see I'm going to need at least one more. I hiked and backpacked in low 60 deg temps with this as my only layer and was completely comfortable. At night when the temp dropped to 40 I only added a fleece. For as thin as it is the warmth was amazing. The next day when it hit 75 I was still fine. I'm 5'7" 145 lbs and bought the small, turned out also to be perfect. Love the extra length on the sleeves too. You can't go wrong with this shirt!