I bought this product to specially use for outdoor, winter running in Chicago. I've used this jacket as a baselayer and often layer a thin vest or windproof jacket over it. I am 5' 11" and 135 lbs and bought this in a Size Small. I generally size down on Patagonia baselayers because I don't need the longer sleeve length and like it to be tight. This shirt is SLIM fit, so unless you have a thin, runners body, or like tight shirts, size up.Pros: "Iron Clad warranty". This shirt is super thin and really warm. It has little hand cuffs at the wrists that you can cover your closed hands with! Its machine washable- cold and line dry. Cons: Expensive (but worth it after you get over the $100+ price tag). The wrist cuffs are awesome, but not too practical as if it's cold enough to run in this wool shirt, then I'll probably have gloves on. Bottom line: You need to test this out if running in it as it will keep you much warmer than you might think. A+ shirt.