A seamless base layer makes for one of the hardest working and most versatile pieces in your wardrobe. If you don't have one, start with Castelli's Seamless Men's Base Layer; you'll barely know you have it on under kit, but the whisper-thin nylon and polypropylene fibers will quietly work throughout your ride to shuttle moisture from your skin to your jersey, keeping you cool and dry on warm days. Keep it in your rotation when the temperatures start to drop and it will begin to act as an insulating layer during chilly mornings and windy descents. Castelli constructs the base layer expertly, as usual, to mimic your body movements in the saddle and remain next to your skin to wick moisture as quickly as possible. It weaves the synthetic fibers in an open mesh pattern to ensure air moves freely throughout the base layer, adapting to changing ride conditions without inhibiting your movement in the saddle.