This jacket is awesome. I am 5'6'', 150 lb, athletic build with about 40" chest and 32" waist. The medium fits like a glove-close fit so perfect to layer under a shell, but too close to layer much under it, so if you want to do that size up like they recommend. Awesome construction, lightweight, super warm, not constricting. I love how Stoic tops fit-baselayers, jackets, etc, perfect for the athletic build types.