I really don't like tight shirts. I usually wear XLT. This XXXL (3XL) fits almost exactly. 2XL is tight. Size XL is ridiculously tight. If you need a compression shirt, you will be satisfied with the size. If you want tight skin, order one size up. If you want to order a t-shirt 2 sizes bigger. The fit of the 2XL Mission Alpha is NOT the same as the 2XL Mission Proton. Proton is a tapered athletic cut. I could not objectively verify the claim of 5x faster cooling of the proprietary 37.5 polyester. Since I didn't burn forty bucks on hype but only spent five, I don't have to believe it. I wore this in 81 degree weather with a strong breeze on the beach mostly in the shade of a canopy. The unprotected heat of the waterline made me sweat but didn't feel uncomfortably clammy. However, I get similar results with DRI Equip polyester fabric used in both Joe's USA and Sport-Tek shirts. The hype around 37.5 could have been bolstered by videos of people in science coats and material comparisons of evaporation times. Lab coats = trust, media propagandists assure us.