Most liner socks are made from manmade materials like nylon, polyester, polypropylene, spandex and a combination of those materials. The reason is simple: Those are the materials that excel at the wicking function mentioned earlier. Regardless of what exterior sock you are wearing, socks made of these materials wick the moisture away from your feet, and that's what keeps them dry. Other liner socks are made of wool, which also has wicking properties, but not as good as the earlier mentioned "smart" materials. When used in a blend with those materials, however, wool can be a good addition to make liner socks that are both functional and comfortable. Also smart wool liner socks, made from treated merino wool, have good wicking qualities and are very warm. Absolutely stay away from any socks made of cotton or with a high amount of cotton in them, since cotton doesn't wick and will hold water next to your skin all day long.