I wear this under my jersey when it is hot, cool and cooler. Prevents the jersey from sticking to me and obviously helps evaporation. I wear a size large jersey and bought this in a medium. Worked out perfectly as far as size and function. I did notice on the first wearing that I thought the armhole was cutting in at the front of my armpit. Don't remember noticing it as much the more I wear it. Anyway, I am very happy I purchased this garment and really only did so because of the print. The base layers that are all white look dingy no matter what you do.