A bag (also known as a sack) is a non-rigid or semi-rigid container, made of paper, cloth, plastic, leather, or some other flexible material.

A bag is used for packaging and/or carrying items. For the latter a bag may have one or two handles; a shoulder bag has a strap to carry it on the shoulder (the bag is either carried on the side of the shoulder carrying it, or the other side). A rucksack has straps to carry it on the back. There is also be a string bag, with two strings which go over the shoulder and can be used as the fastening as well.

A bag may be closable by a zipper, snap fastener, 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.

A bag may or may not be disposable; however, even a disposable bag can often be used many times, not only for economic but also for environmental reasons. On the other hand there may be logistic or hygienic reasons to use a bag only once. For example, a garbage bag is often disposed of with its content. A bag for packaging a disposable product is often disposed of when it is empty.

An empty bag may or may not be very light and foldable to a small size. If it is, this is convenient for carrying it to the place where it is needed, such as a shop, and for storage of empty bags.

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.

The word probably has its origins in the Norse word baggi. Although paper had been used for purposes of wrapping and padding in ancient China since the 2nd century BC,^[1] the first use of paper bags (for preserving the flavor of tea) in China came during the later Tang Dynasty (618-907 AD).

There are environmental concerns regarding use and disposal of plastic shopping and trash bags. Efforts are being taken to control and reduce their use in some European Union countries, including Ireland and the Netherlands. In some cases the cheap bags are taxed so the customer must pay a fee where they may not have done previously. Sometimes heavy duty reusable plastic and fabric bags are sold, typically costing €0.5 to €1, and these may replace disposable bags entirely. Sometimes free replacements are offered when the bag wears out. A notable exception to this trend is the UK, where disposable plastic bags are still freely available and are dominant.