A sleeping bag is an insulated covering for a person, essentially a lightweight quilt that can be closed with a zipper or similar means to form a tube, which functions as lightweight, portable bedding in situations where a person is sleeping outdoors (e.g. when camping, hiking, hill walking or climbing). It is also commonly used indoors for people who do not have beds or at sleepovers for when one or more persons cannot all fit in the bed or do not feel comfortable sleeping with someone. Its primary purpose is to provide warmth and thermal insulation through its synthetic or down insulation. It also typically has a water-resistant or water-repellent cover that protects, to some extent, against wind chill and light precipitation, but a tent is usually used in addition to a sleeping bag, as it performs those functions better. The bottom surface also provides some cushioning, but a sleeping pad or camp cot is usually used in addition for that purpose. The bottom surface of a sleeping bag may be moderately water repellent, but a plastic tarp or groundsheet is often used to protect against moist ground.

There are a range of sleeping bag models designed for different purposes. Very lightly insulated sleeping bags are designed for summer camping use or for indoor use by children during slumber parties. Well-insulated bags are designed for cold weather use. The most well-insulated and lightweight sleeping bags, which are designed for serious hikers and adventurers, are more expensive than lightly insulated sleeping bags. One subcategory of cold-weather sleeping bag, the mummy bag, is so named because it has an insulated hood for the head. A bivouac sack (bivy) is a waterproof cover for a sleeping bag that may be used in place of a tent by minimalist, experienced hikers. A bivy bag may also be carried by day hikers as a backup or emergency shelter, to be used if they cannot make it back to their starting point by nightfall due to inclement weather or getting lost.