Very thin. My 10 year old son normally wears a size L 10/12. I ordered this in L and it is WAY too big. Looks like a mini dress on him. I can tuck it in for baseball but that does nothing for the looseness of the neck and shoulders. I needed it that night so couldn't return. Even if the shirt had fit well, Im still disappointed in the quality as it is very thin material. No thermal property to it. Although it does dry quickly.