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Thermal underwear likely reminds you of the pajama-like long-sleeve cotton shirts and bottoms you may have worn as a child. While the function of long underwear remains the same—keeping your body warm and insulated—thermal underwear has come a long way since then. Merino wool and fully synthetic long underwear options provide thin but effective layers of insulation. Synthetics also provide heat-trapping and moisture-wicking qualities. You can layer thermal underwear under your clothes, but you can also wear thermal leggings alone for a stylish, form-fitting look.

High-Performance Leggings

These mid-weight leggings are made of 100% high-quality wool, making them warm, durable, and comfortable. Smartwool

Tip: Consider the fabric. Merino wool is one of the best materials for thermal underwear. It’s warm, moisture-resistant, and much comfier than products made out of normal wool. It’s also lightweight, keeps its shape, and doesn’t retain odors.

Fashion and Function

These bottoms are thick enough to wear around town and have an optional pouch to stash your phone. BALEAF

Tip: When you’re on the move in the winter, you need bottoms that are both flexible and warm. Thermal leggings are a great option, especially ones that are made with stretchy, synthetic fabrics. Thermal leggings are generally pretty thick and will trap heat against your legs without being see-through. The form-fitting look provides insulation—but still works as stand-alone pants.

Whole Body Warmth

The matching, ultra-soft shirt and leggings will help you preserve heat in your entire core, keeping you comfortable in frigid conditions. Nooyme

Tip: While leggings might be immediately what come to mind when talking about thermal underwear, you should also consider getting a base layer for your entire body. You can purchase a full-body thermal underwear suit, or you can mix-and-match thermal leggings with a thermal shirt. If you’re taking this approach, you should buy a similar shirt to match your leggings so you’ll be evenly insulated—and there’s no better way to accomplish that than by buying a matching set.