Normally wear medium or large.  Got the XL based on reviews.  I would say that fits like a medium.  The problem is the buttons are super small so if you are slightly active they start popping out of the holes if you don't wear three sizes too big.  Then you look like a creeper who wears his shirt unbuttoned.  Not a good look if you aren't cool with the child molester look.  When it's functioning properly it looks pretty cool and actually keeps me warm hence the three stars.