As your body emits energy, the mineral-infused fabric absorbs & reflects it back into tissues & muscles, improving endurance & strength. We sent 13 testers out in the winter to hike, ski, shovel snow, and run, and we found a variety of styles, fabrics, and cuts to keep all types of bodies warm. We have picks that will take you from city to trail, picks for hiking and backpacking, and picks for fishing and hunting—or even for skiing and snowboarding. We also have ideal options for getting muddy at sports practice or for layering under your civilian clothing. And, of course, we have a budget option. In the title of this guide, we call these layers “thermal underwear” because that’s what most people search for when they’re looking for something to be active in. And that’s what base layers are—a clothing layer that regulates body heat while you work up a sweat. If you’re looking for loungewear or pajamas, we have a guide to those, too.