Because your body temperature will fluctuate depending on the weather and your physical activity, it helps to wear multiple lightweight pieces. Even REI experts agree that if you need to buy one layering piece, the Smartwool 250 Midlayer Zip T Top performs the best. It’s made of high-quality merino wool from New Zealand, which gives you a soft, breathable, moisture-wicking, and odor-resistant layer to keep you warm and dry. For colder temps, it’s also light enough to easily layer under a fleece, down jacket, or vest. Snag the corresponding 250 mid-layer merino bottoms for men and women for the full body set. As a bonus, the bottoms can double as an extra warm layer underneath jeans on cold wintry days in everyday life.