Castelli have re-engineered the traditional base layer to make this, the Core Seamless Base Layer perform better than ever. Many seamless base layers use a mixture of Lycra & Nylon but these hold a lot of moisture. Not good for comfort. Instead, Castelli uses a blend of polyester & polypropylene yarns to quickly move moisture off the skin with very little moisture uptake. This offers excellent temperature regulation while also being extremely soft & comfortable against the skin. It's not for the very hottest of cycling conditions but in cooler & mild conditions, worn as your first layer under long or short sleeve jerseys, it really does excel.