Base layers, thermal underwear, long johns-whatever you want to call them, nothing in the process of layering clothing for warmth is as important as the layer that touches your skin. This layer has to do double duty, insulating you but also allowing heat and moisture to pass through it and out, away from your body. To find the best base layers for different scenarios, we've spent years testing them while hiking alpine lakes in the summer, snowshoeing through the winter, and dozens of trips between those extremes.