I am 5'9'', 160 lbs and use a large. Loose fit. I run/jog in this shirt. For this, it feels great in the peak of hot/humid summer in the south. It performs as advertised: wicks moisture, blocks sun, allows ventilation and is very cool and comfortable on the skin. It feels great alone in the fall and spring, as well. I like the reflective logos; maybe it helps a little so cars can see me at dusk. I also take extensive trips into various backcountry environments with this as my primary shirt. It layers well with capilene products. It acts like a cap 2 short sleeve yet is cheaper. After taking on significant moisture, it dries quickly even in humid environments. It piles around backpack harness/belt areas but doesn't really impact the performance or integrity of the garment. The piling doesn't look that bad, but worth mentioning in case you want to wear this all the time. I do not machine dry this shirt. The white color offered with this garment is not really white, but more like an off-white. The fabric does not seem to keep odors after many days of physical activity, either. In my opinion this is a great shirt for an array of physical activities and cheaper than many name brands on the market today. This is all I know; testification complete.

Like most other reviewers have said, this shirt runs a little large. Not in length, just in width. For us lanky dudes it's not a problem if the shirt is worn on its own, but I won't be trying to layer it under anything. The fabric is not as smooth a surface as my other tech tees... it reminds me of those disposable covers they put on airplane pillows. Wicks, breathes, and gets the job done though.

I use these as a base in the winter and everything in the summer. Good in the Gym, on the river, mountain, and running. And the best part is the fit... they don't make you look like a tool trying to show off... relaxed fit.

It fits and feels great. Can't wait to wear it out on the trails! The fit is about average, I'm 6'6 and the XL is a little snug and the XXL is perfect, so size accordingly. . .

I'm sure its a great shirt. I'm a huge Marmot fan, but all their clothes run big. I don't understand. Got this shirt in Medium and its the same size as my large T-shirts. Fits well in the chest and shoulders but the sleeves are too long. I'm gonna send mine back and try something else.

I ordered a medium and it was still loose. Usually wear a large for normal fit and a medium for tight. This is definitely not one of the form-fitting-type undershirts. Good shirt though!

A little thicker than I thought it would be. still a nice shirt lg is a little big/ loose 5' 9" 165lbs

Great shirt for form and function, but got a run in it the second time I wore it and soon developed a hole.