What could possibly keep you warm until you warm up, then keep you comfortable when you start sweating on the trail? The men's tall REI Co-op lightweight long-sleeve base layer crew is what.

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Comfy as you warm up, comfy as you cool down, even comfy when you break a sweat—the women's REI Co-op lightweight long-sleeve base layer top is ideal for skiing, hiking and lunch break, too.

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The women's REI Co-op long-sleeve lightweight base layer crew top wicks, breathes and stretches for comfort through every temperature change as you hike or ski the day away.

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The REI Co-op lightweight base layer zip-neck top in men's tall sizes knows how to keep you comfortable through a whole day of changes from cold to warm to sweaty and back to cold again.

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Start layering your little ones with the toddlers' warm, breathable REI Co-op Lightweight Crew Long Underwear top. It features a made-to-move design and all of the comfy for a day of play.

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Start layering your littles with the kids' warm, breathable REI Co-op Lightweight Crew Long Underwear top. It features a made-to-move design and all of the comfy for a day of play.

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When it comes to base layers, basic is better. The men's Lole Logan half zip is a long-sleeve base layer top that's perfect under your gear for outdoor training or activities, or for everyday life.

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Providing lightweight warmth and natural temperature regulation, the Kari Traa Lucie Long-Sleeve base layer top is made from fine merino wool and designed to layer smoothly under your jacket.

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Whether you wear it on its own or as a base layer, the women's Smartwool Merino 150 Lace V-Neck short-sleeve top is designed for comfort when you're on the move.

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Hike, bike and even sleep in the men's REI Co-op Merino 185 short-sleeve base layer top. The soft, breathable and naturally odor-resistant ultrafine merino wool makes it an ideal next-to-skin layer.

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Soft, breathable and naturally odor resistant, the women's REI Co-op Merino 185 short-sleeve base layer top is an ideal next-to-skin layer for hiking, cycling and even sleeping.

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The women's REI Co-op Merino 185 short-sleeve base layer top is an ideal next-to-skin layer for hiking, cycling and even sleeping, thanks to the soft, breathable, naturally odor-resistant merino wool.

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Soft, breathable and naturally odor resistant, the ultrafine merino wool of the men's REI Co-op Merino 185 long-sleeve base layer top is beloved while sweating on the trail—and sleeping in the tent.

Underwear TopsBase Layer Tops

Soft, breathable and naturally odor resistant, the ultrafine merino wool of the men's REI Co-op Merino 185 long-sleeve base layer top is beloved for both sweating on the trail and just sleeping.

Underwear TopsBase Layer Tops