I bought these shirt as school uniforms tops. We live in Houston where it's hot a humid and my boys like the dry-fit shirt and these were perfect. I typically buy youth XL or adult smalls but the sizing was hard to interpret. Using the chart (plus emailing the company to find out how to measure), I decided on ordering all mediums and ended up having to exchange half of them. I have twin 12 year old boys and one is bigger than the other so they typically require different sizes. One is about 5'5 and I eventually settled on a Large but I think an XL would has been better. Smaller boy is 5'2' and the M fit him, but the Large would have been fine too. Both boys are slim. If I had to re-order, I would have gone with the Large and XL.