Ideal for cold weather, the kids' REI Co-op merino midweight base layer bottoms are soft—not scratchy—naturally regulate temp and made with wool that's certified to the Responsible Wool Standard.

Underwear BottomsBase Layer Bottoms

Oh so soft and not itchy, the kids' REI Co-op merino midweight base layer top is an ideal first layer for cold weather. It naturally regulates temp and is certified to the Responsible Wool Standard.

Underwear TopsBase Layer Tops

Your underwear should deliver lasting comfort. Whether you're spending the day at work or a couple hours at the gym, Smartwool Merino Plant-Based Dye boxer briefs help you stay confidently comfy.

UnderwearUnderwear Bottoms

A wide, merino-lined elastic waistband and flatlock seams help make Smartwool Merino Print boxer briefs your new favorite for any activity—from urban exploration to new trail adventures.

UnderwearUnderwear Bottoms

Built with the outdoor enthusiast in mind, the men's Smartwool Merino Sport long-sleeve quarter-zip top provides thermoregulation, odor resistance and durability for years of adventure.

Underwear TopsBase Layer Tops

Avoid panty lines on the trail in the soft, breathable and naturally odor-resistant REI Co-op Merino Merino Thong Underwear for women. They're made to keep you comfy throughout the day.

UnderwearUnderwear Bottoms

Keep the winter chill away with the women's Icebreaker MerinoFine base layer leggings, which feature a high waist for added coverage—and its merino wool is naturally stretchy and odor resistant.

Underwear BottomsBase Layer Bottoms

An ideal performance layer, the men's REI Co-op Midweight Base Layer Bottoms boast a smooth face and low-bulk seams for easy layering and a brushed back for added warmth and comfort.

Underwear BottomsBase Layer Bottoms

Stay dry, warm and stoked all day with the men's Burton Midweight Base Layer Crew top, which gives you moisture-wicking fabric and comfortable, chafe-free seams.

Underwear TopsBase Layer Tops

Hard-charging winter days start out cold, but you always warm up. The men's REI Co-op Midweight Base Layer half-zip top offers the warmth of long sleeves and the easy venting of a front zipper.

Underwear TopsBase Layer Tops

The women's REI Co-op Midweight Base Layer Half-Zip Top offers warmth in the cold with long sleeves and easy venting thanks to a front zipper.

Underwear TopsBase Layer Tops

Stay dry, warm and stoked all day with the men's Burton Midweight Base Layer pants, which give you moisture-wicking fabric with a cottonlike feel and a plush waistband.

Underwear BottomsBase Layer Bottoms

Start with a high-performing next-to-skin layer and take advantage of the snow. The women's REI Co-op Midweight Base Layer Tights have a smooth face for easy layering and brushed interior for warmth.

Underwear BottomsBase Layer Bottoms

Make your kiddo's base layer the warm, breathable, kids' REI Co-op Midweight Crew long underwear top and they'll play happily outside all day. It's great on its own for inside play, too.

Underwear TopsBase Layer Tops