I use this jacket with a thin wool base layer and a mid weight wool base layer. It keeps me warm and breaths well too. The medium fits me perfectly; I am 5'4" and about 135 pounds. It is supposed to fit snug. It has three large pockets in the back. The only complaint I have is that the wrist bands are loose and let in cold air; they are also difficult to tuck into my winter gloves. I wouldn't use this jacket for much warmer than 35 degrees as it might get too warm.