I am 5'6" and a slender 160lbs.  I am usually a medium in tops and medium or large in bottoms. I went with the medium and thought it fit really good.  But a month later, at a Scrub-hub store, I bought the exact same shirt in small and it fits better.  I liked this shirt over the other white under-scrub shirts I've tried because the neckline is a little wider and doesn't peek out from the neckline sides of the scrub top I have and the fabric just feels really nice.  I have a compression top that I used to use before this shirt and liked the tightness but found it's hard to scrunch up sleeves before starting procedures (like IVs) where I don't want to get anything on the ends of the sleeve.  This shirt's fabric is soft and stretchy and easy to pull up on forearms.  I've had the medium for a few months now and it's held up really well.