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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Keep your temperature just right on cold days with the built-in ventilation of the women'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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This minimalist Smartwool Intraknit Thermal Merino women's crew base layer top is lightweight and super-moveable—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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Highlight your base-layer comfort in floral. The women's Smartwool Intraknit Thermal Merino Pattern crew top is designed with targeted mesh-ventilation and a 3D-mapped fit to venture through the day.

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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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A good choice for cool-weather adventures, the women's Smartwool Intraknit Thermal Merino quarter-zip base layer top helps keep your body dry and comfortable—and the fit won't slow you down.

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Spun from superfine merino wool, the women's Kari Traa Lam long-sleeve base layer top is exceptionally soft, smooth and breathable for easy layering year-round.

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Combining a warm, breathable 100% merino wool exterior with a 100% LIFA® wicking interior, the women's Helly Hansen LIFA Merino Midweight crew top is the perfect base layer for cold-weather pursuits.

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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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Comfy as you warm up, comfy as you cool down, comfy 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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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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