The best base layer and thermal underwear provide optimal protection to your body when skiing, running, mountaineering, going to work, or even watching football games in extreme cold.

How long underwear is useful: Long underwear works as a thermal and wicking layer. It keeps you warm while not compromising on  fit and comfort.

The best layer and thermal underwear top (for women): Trew Lightweight NuYarn Merino ¼ Zip. (form fit, soft fabric, good styling, long sleeves, and proper torso length)
The second best layer and thermal underwear top (for women): Women’s Duckworth Comet Hood
The best affordable layer and thermal underwear top: REI Midweight Long Underwear Crewneck Shirt (men), REI Midweight Space Dye T-Zip (women)
The best layer and thermal underwear top (for men): Icebreaker Men's Oasis Long Sleeve Half Zip with Hood
The second best layer and thermal underwear top (for men): Men’s Smartwool NTS 250 Zip
The best layer and thermal underwear bottoms (for women): All the Right Places Pant II (with elastic pockets)
The best layer and thermal underwear bottoms (for men): Men’s TASC Level B Base Layer Pant
The second best layer and thermal underwear bottoms (for women): Women’s TASC Level B Base Layer Pant
The best affordable layer and thermal underwear bottoms (for men): REI Men’s Long Underwear Midweight Bottoms