A bag is a container that is usually used for storing or holding something. Bags are non-rigid, normally made of paper, cloth, thin plastic or some other flexible material. A bag may have one or two handles, or none at all. A bag may be closable by a zipper, etc., or simply by folding (e.g. in the case of a paper bag). Sometimes a money bag or travel bag has a lock. Sachets may be fabric bags that are filled with pot pourri and tied off with ribbons.

The word probably has its origins in the Norse word baggi.

Bags vary from small ones, like purses (e.g. as an alternative to putting small objects in pockets of clothing) to large ones for use in traveling like a suitcase.

Cheap disposable paper bags and plastic shopping bags are very common in the retail trade as a convenience for shoppers, and are often supplied by the shop for free or for a small fee. Customers may also take their own shopping bags to the shop.