Adventure all day in the cold with the enhanced breathability of the men's Smartwool Intraknit Thermal Merino Colorblock Crew base layer top, designed with body-mapped mesh and distinctive patterning.

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Keep your temperature just right on cold days with the built-in ventilation of the men's Smartwool Intraknit Thermal Merino Colorblock Quarter-Zip base layer top featuring body-mapped mesh zones.

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This minimalist Smartwool Intraknit Thermal Merino crew base layer top is anything but insignificant. It's a lightweight, super-moveable top perfect for all of your sweatiest cold-weather activities.

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Whether you're skiing, ice-fishing or snowmobiling, set the tone for all-day comfort with the body-mapped mesh vents of the men's Smartwool Intraknit Thermal Merino Half-Zip base layer hoodie.

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Engineered to enhance performance and maximize mobility, the men's Smartwool Intraknit Thermal Merino Quarter-Zip base layer is durable and warm yet lightweight and easy to layer thanks to a slim fit.

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Layer up for sledding, skating or shredding in the men's Helly Hansen Lifa Merino Midweight Crew base layer top. Its 2-layer construction combines cozy merino wool with durable technical fabric.

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Heading out to play hard? The REI Co-op lightweight base layer crew top keeps you warm until you warm up, then wicks sweat to maintain your cool when you're charging.

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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 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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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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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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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 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.

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