I read other reviews so ordered a size up. I got the large for my very slim, somewhat tall 10 year old and they fit him perfectly. He loves them and only wants to wear these. He always chooses them over his nice cotton boxer briefs (that I spent a fortune on from Hannah Anderson). In fact, he likes them so much that "more sports underwear" is the top thing on his list of things he wants for his 11th birthday! So, here I am buying more and writing a review of boys' underwear.