i just bought the jacket and was having the same trouble deciding the size. i am a little under 5'8 and 130 pounds. i pretty much always always wear a small. so therefore i went with the small. the jacket just got here. it seems to run pretty true to size.... but theyve made it to be a snug jacket as to layer under things.. i dont like things to be fitted or snug, so i believe i am going to return mine and get a size up, a meduim. if you want a base layer jacket, a jacket that fits like a glove, order your normal size. If you want to layer one or two things under it, or just like jackets to be a little roomy get a size up. if it helps, i wear size small in columbia and NF, and that fit is perfect, not snug, not baggy. My mountain hardwear monkey jacket small runs more fitted than the columbia and NF small. hope it helps