Layering clothing allows a cyclist to keep their core body temperature consistent throughout increased activity levels, and despite outside tempertaures and conditions, such as wind, rain and snow. The concept behind layering apparel is to control core body temperatures by peeling off layers of clothing as you heat up, and by adding layers of clothing as you cool off. There are three catagories of layering- a base layer designed to wick moisture, middle layer designed to insulate the body, and an outer layer designed to protect the cyclist from the elements.

The "base layer" refers to the layer of clothing that is right next to your skin. The purpose of the base layer is to manage moisture. A cyclist's core body temperature heats up as the level of activity increases. The body begins to sweat in an effort to cool the cyclist down. Sweat trapped against the skin leaves the cyclist feeling cold and wet even with an outer shell designed to protect against the elements.

In order to keep skin dry, base layers are made of fabrics that are designed to pull moisture away from the skin and transfer it to an outer absorbant layer where it can evaporate. Keeping dry allows the cyclist to maintain a cool body temperature when it is hot, and avoid hypothermia when it is cold.

Most cycling base layers are form fitted in order to allow for greater freedom of movement and easier layering. Base layering pieces include bra's, briefs, tights and shirts.

Base layers are available in a range of fabrics, each designed to match the cyclist's level of activity and outside temperatures and conditions.
More Info on Base Layer Fabrics

The insulating layer refers to the layer of clothing that goes over your jersey or short, and is designed to keep you warm. Styles usually include fleece tops and vests. Insulating layers should be made of warm, lightweight fabrics such as technical fleeces and wools. Fabrics should breathe well so that sweat and body heat escape, keeping the cyclist warm and dry. Most insulating materials come in various weights which impact the degree of warmth they offer. These garments should have a loose fit for good freedom of movement, but they should also fit under outerwear without bunching.