For the better part, rash guards are worn as is. You do not need to put anything else underneath the rash guard. Since it fits so snugly, adding something underneath would mean that you are destroying the fit of the item since it is made of really stretchy fabric. However, if you are in cold temperatures, it will help to wear a wetsuit on top of it. The wet suit is designed to keep you warm, and the rash guard will just be an extra layer of warmth for you. For the better part, rash guards are worn with biker shorts or swimming shorts. As long as you ensure they are not too tight on you to move around freely. A fitting rash guard will not have enough allowance for you to wear anything underneath, so keep that in mind. It will make wearing and styling it easier for you.