I'm 5'9", 150 - 155 lbs, and wear a 41 reg on jackets. The fit on the Alder in medium is spot on. It's got a good-looking, clean, modern cut that doesn't have much extra room or bagginess. With that said, if you like to wear your casual jackets a bit more loose, or were planning to wear it w/ a heavy sweater or sweatshirt, I don't think taking an Alder one size up would look ridiculously big, or feel overly cumbersome. It's got a light lining, which I estimate is good enough for 85% - 90% of the "cold" temps we see here in the coastal communities of Southern California w/o needing to wear anything remotely heavy underneath. I'm pretty sure wearing just a t-shirt and the Alder would be good enough for temps of 50° F or warmer. This is not just a shell. This is not a jacket I would wear on a cool, breezy summer night on the beach, because it would just be too warm for that. But I can see why most people would classify the Alder as a "light jacket".