A base layer is designed to wick moisture away from your skin. That means you stay comfortable when you're working hard and perspiring. More importantly, you don't get chilled when you stop. In summer, wear a base layer on its own. When the weather takes a turn for the worse, you can add a fleece or other wicking mid layer for insulation. A breathable windproof or waterproof outer layer completes the system and will keep you warm and comfortable in all conditions.