A base layer (aka long underwear or thermals) is the shirt and pants that you wear as your first layer of clothing. It often is close fitting and meant to keep you warm when the temperatures start to plummet. It's essential for cold weather hiking and useful in the swing season when temperatures are fluctuating. In the warmer weather, a base layer is ideal as a dry layer for sleeping. Merino wool is a natural fiber harvested from Merino sheep. Though Merino sheep originated from Spain, almost 80 percent of merino wool now is obtained from sheep raised in Australia and New Zealand. Merino wool is the gold standard for base layers because of its has many desirable qualities WARMTH: Like most wool fabrics, merino wool is extremely warm. The individual fibers are slightly crimped, creating air pockets that trap warm air. It makes an excellent base layer for winter and swing season outings. BREATHABILITY: Merino wool fibers are among the most breathable and moisture-wicki