I wear this when I work outside in weather in the 30's to 50's (about 8 hours) and I have to say, the top isn't as good as the baselayer pants. The arms are snug, but there's extra room around my torso that I think my torso could wear an XS but I don't think the arms would fit. Admittedly my arms a little thick for my size, but I still think the XS top may have had room in my torso also.  When I'm doing physical work, I can be warm wearing just this top and a t-shirt over it in upper 40's and 50's weather. But if I'm just walking around or doing light physical work, I still need at least one jacket. When it's lower 40's or 30's, I usually am wearing 2 jackets (one also has omni-heat) and I can still be cold if it's windy at all.  Without a jacket on, it's a weird feeling in that I can feel the cold weather but also feel the heat all at the same time so I'm both cold and warm on my arms. I like wearing this when I know it's gonna warm up during the day but if it's gonna be cold the whole day, I have an actual compression shirt that works better.