When we're training and racing during the spring months, the last thing we want is sweat held in fabric, keeping us too cold for the descent back down the canyon. Castelli's Core Seamless Baselayer uses a mix of polyester and polyproylene which wick moisture away from the skin better than traditional nylon and Lycra constructions. The cut is form-fit to keep those moisture-wicking properties where they're needed, and the sleeveless design layers even better under jersey or jacket.