Serious sleeping bags (i.e. those intended for outdoor camping) are rather incredible feats of human innovation. Compact, reasonably durable and wonderfully warm, sleeping bags make it possible to truly enjoy sleeping outdoors. In essence, these bags are simply blankets that can be secured around an individual's body by means of a zipper or other similar device. In practice, sleeping bags are specially designed pieces of outdoor equipment that provide an incredible amount of warmth without being overly heavy or bulky.

The shell of a sleeping bag designed for camping is usually made of nylon or polyester. These fabrics have the advantage of being extremely pliable and lightweight, making the bag easier to carry. The shells of best sleeping bags are treated with water proof coatings. The inside of a sleeping bag is sometimes lined with a different material, such as polyester taffeta, which makes the bag more comfortable to use. But the most part of a sleeping bag is what is in between the shell and the lining: the fill.

The fill of an outdoor sleeping bag is traditionally made of bird down. Down feathers, which are found under a bird's exterior layer of feathers, are extremely warm and provide excellent thermal insulation. Some of the most expensive sleeping bags use goose down, which some experts hold to be the best. However, not all modern sleeping bags use down. Some feature synthetic fill, such as polyester fibers.

There is some debate of what kind of fill is the most effective, and opinions vary according to individual. One the one hand, synthetic fill does not readily absorb water, dries easily and provides some warmth even when completely soaked. Under extreme circumstances, these properties can save the life of a camper. Synthetic fibers are also resilient, which means that they insulate well even when being depressed by a person's weight. However, synthetic fill is not as compact, so the bags tend to be relatively bulky, making them more difficult to carry. Down fill, on the other hand, is extremely compact and lightweight. It is also technically better at retaining heat than synthetic fibers, at least as long as it is dry. However, if it becomes wet, then it loses all ability to act as insulation.

Most modern outdoor sleeping bags are vaguely shaped like the human body; that is, they are narrower at the base and wider at the top. This shape reduces both the bags volume and its surface area, making it easier to carry and easier for the body to warm up. Some bags are even specifically designed to fit women, who obviously tend to have wider hips and narrower torsos than men. Most outdoor sleeping bags have a drawstring around the top, allowing the sleeper to completely encase him or herself within the bag. Alternatively, some bags have an actual hood at the top. The hood is a nice feature, as it allows the sleeper to cover his or her head while remaining in a natural sleeping position. In a bag without a hood, the sleeper must usually curl up in order to fit inside.