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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Make your first layer a comfortable one with the men's Picture Organic Clothing Lhotse base layer pants. They feature chafe-free seams and a nonirritating elastic waist band.

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Fend off the chill and stay out longer in the men's Helly Hansen Lifa Merino Midweight base layer pants. Built for winter fun, they blend merino wool with technical fabric to keep you cozy and warm.

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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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These REI lightweight base layer bottoms are designed to keep you warm and wick away moisture during high-performance outdoor activities when the temperature starts to drop.

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From morning's first frosty steps to midday sweat and exhilaration, the men's REI Co-op lightweight base layer bottoms wick and breathe to keep you comfortable all the way.

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The men's REI Co-op lightweight base layer bottoms wick and breathe to keep you comfortable from the first cold steps of morning to sweaty exhilaration on the trail hours later.

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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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