What should I wear under my cyclocross jersey for cold races?

A thin synthetic or merino wool base layer is ideal—something that wicks moisture but adds a bit of warmth. Avoid cotton, which holds sweat and makes you cold. For very cold races (below 5°C), consider a slightly heavier base layer and add arm warmers. The key is to dress for how warm you'll be at race effort, not how cold you feel at the start line—you'll be generating significant heat within the first minute.