How do the sizes run? Large small?

I am giving this long answer, because sizing can be such a difficult hurdle in ordering activity-specific gear online. Hope it helps those who read the reviews moving forward. I do own this jacket and wear a medium at roughly 5'11 170lbs. I do not use this as a belay jacket. They run small - as a rule UK brands will fit a half-size to a full-size smaller than their US equivalent(s). For instance, in a Rab Shadow Hoody, a midlayer piece, I wear a large, which is unheard of for me. In a US brand, like patagonia, I would wear a medium for a comparable jacket. That Large fits perfectly (not tight and not for more than a small layer or two underneath) - the same goes for the length of the sleeves and torso. They fit well, nothing looks out of scale with my dimensions. So, for me: UK midlayer = size large. Keep in mind that the Montane Prism 2.0 is a belay jacket - a packable, lightweight layer that stays in the pack and comes out when you need to stop climbing and will be standing in the elements without creating much heat. SO- this being UK sizing, but specific to belaying makes the fit larger than a jacket that may be intended for base or under-shell layering, like per-say a fleece or softshell. What that means is that if you wanted to wear this as a casual winter jacket or an under-the-shell layer for skiing etc. then you would want to order your normal US jacket or shirt size. Then you would have a close fitting, yet comfortable basic insulating jacket which forgoes the function of being able to throw it on over everything else when belaying. So for me: UK belay jacket used as stand-alone jacket = size medium. To clarify, If I were wanting to throw this over a shell/ mid-layer/base-layer; I would have ordered a large.