What does a base layer do? Think of it as a comfort layer and you wont be far off the mark. They can wick sweat from your body when the weather is hot. They can trap a layer of warm dry air next to your skin when its cold. They can retain warmth even in pouring rain, they can block the wind, hey they even work in tandem with outer layers creating a unique personal atmosphere. Cool in the heat, warm in the cold, wind blocking performance.... whatever your needs Castelli has all bases covered.