Also known as an undershirt or vest for the jargon-averse, getting the right base layer against your skin can make a big difference to cycling comfort all year round. In winter you wear more layers, and arguably the most important layer is a base layer. How many layers you wear over the top is down to riding conditions, temperature, weather, duration, intensity and personal preference. Start with a good base layer and you’re off to a good start to cycling happily through the winter.
A base layer isn't quite as important in the summer, but can still make a useful contribution to your on-bike comfort. A lightweight base layer will help move sweat away from your skin, a job some jerseys don't do quite as well as you might hope. And there's another reason why you'll often see pros wering base layers even in the height of summer: if you crash, your jersey will slide over your base layer, reducing damage from abrasion.