Despite seeing that on the Ex Officio size chart I should wear a size large, I followed the advice from a few reviewers and ordered a size down - but it was way too snug. I think it depends on how you plan to wear it. If I was going to wear it as underwear, maaaaybe a size down would be okay. I normally wear a large, and the medium was too tight. I don't think it'd be comfortable even as a base layer, but I plan to wear this as a sleeveless top by itself when I'm traveling in the tropics. I sent it back and am awaiting the size large. Just follow the Ex Officio size chart with your measurements.

The material is fast drying which is great when you're in a hot climate and sweaty, or just if you want to wash and wear quickly.