Comfy as you warm up, as you cool down and when you break a sweat. The women's REI Co-op lightweight base layer top is ideal for hiking, cycling and even sleeping—especially in the summer.

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First half-mile cold; next 3 miles sweaty; lunch break cold. Don't stress—the REI Co-op lightweight half-zip long-sleeve base layer wicks and breathes to keep you comfortable all day.

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The women's REI Co-op lightweight half-zip base layer top wicks and breathes to keep you comfortable hiking or skiing as you go from cold to warm to overheated and back.

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The women's plus-size REI Co-op lightweight half-zip base layer top wicks and breathes to keep you comfortable cold-weather hiking or skiing, as you go from cold to warm to overheated and back.

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The men's REI Co-op lightweight long-sleeve base layer crew keeps you warm until you warm up, then wicks sweat to keep you comfortable when you're going hard.

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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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The women's REI Co-op lightweight base layer tights keep your comfort level high from the first cold steps of morning to the sweaty joy of working your way up a steep trail.

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The women's plus-size REI Co-op lightweight base layer tights warm you in the morning chill, then wick and breathe to keep you cool as you work your way up a hot afternoon trail.

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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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Layer your little one with the REI Co-op lightweight long underwear bottoms for infants and toddlers. Designed for warmth, these bottoms are easy to pull on and absorb moisture as it comes.

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