I was looking for a relatively form-fitting jacket to layer a base layer and long-sleeved jersey under - and in a pinch, maybe even a thin fleece. I'm 5'10", 130lb, and chest 36 so I figured the small would be perfect. It would be fine if worn as the sole layer (though already snug in the shoulders), with plenty of arm length, but got pretty constrictive (like a compression fit) as soon as a long-sleeved jersey was layered underneath. Tried the medium and it's the right one: won't bunch up or flap in the breeze, just has that little bit of give. Haven't tried it in very cold weather yet but it's been great so far.