The base layer is often known as the wicking layer. Wicking is the garment's ability to pull moisture away from the body to the outside of the garment. Even a light sweat induced by moderate activity can produce enough moisture to cause hypothermia. With an effective wicker, moisture is pulled away from the body to the outside of the garment where it evaporates quickly. This leaves the user warm and dry. In cold weather it is extremely important that moisture is pulled from the body even when you feel HOT! Once you stop exerting energy, any moisture remaining on the surface of the skin causes the body to cool extremely fast, and causes the person to chill, maybe even becoming hypothermic. There are many different types of base layers: Capilene (a Patagonia synthetic), Polypropelene(Helly-Hansen, among others), Thermax (DuPont), Thinsulate, and Silk. All of these wick moisture, but some come in varying weights providing more warmth or thermal properties as they are wicking. Silk is relatively the lightest layer, while Capilene, Polypropelene, and the others all have a range of weights. The question may arise as to which of the weights to choose from? For example, if you were cycling in warm to cool weather, a silk or lightweight synthetic (polypropelene or capilene) would be appropriate. On the other hand, if you were backpacking in the Rockies during the winter a heavyweight garment like the heavy-weight PowderStretch® would be appropriate as a base layer. Cotton (the waffle-style print sold in stores as long-johns and any cotton T-shirt) should never be used a layering piece. Cotton will never wick and possesses no thermal value when wet. It increases the risk of hypothermia by allowing heat to continually escape from the body. Cotton absorbs moisture and does not have any wicking capabilities. Once cotton becomes saturated it stays wet for a long time and the wearer expends precious body heat trying to warm this cold, clammy layer covering the skin. Essentially, wet cotton robs body heat in situations where the main objective is to retain body heat.