Don't make the mistake of thinking a base layer is a luxurious extra you can take or leave. Sure, it's not as essential something like a helmet, but the right one can make a pretty significant difference in your ride comfort. Grab Castelli's Seamless Men's Base Layer to see what all the fuss is about and pair it with a regular jersey on warm days and your thermal jersey on colder ones. It'll keep moisture moving away from your skin quickly as you sweat, while acting as a breathable insulating layer during colder days. Castelli uses a blend of nylon and polypropylene fibers to achieve this, cut in a form-fitting silhouette that will mimic your body movements in the saddle. The closer it fits, the better it works. Castelli added a few strategic mesh panels throughout to usher in extra airflow, while knitting the fibers in an open weave to ensure you get consistent ventilation as you ride.