A midden is an old dump for domestic waste. It may consist of animal bones, human excrement, botanical material, mollusc shells, potsherds, lithics (especially debitage), and other artifacts and ecofacts associated with past human occupation.

These features provide a useful resource for archaeologists who wish to study the diets and habits of past societies. Middens with damp, anaerobic conditions can even preserve organic remains in deposits as the debris of daily life are tossed on the pile. Each individual toss will contribute a different mix of materials depending upon the activity associated with that particular toss. During the course of deposition sedimentary material is deposited as well. Different mechanisms, from wind and water to animal digs, create a matrix which can also be analysed to provide seasonal and climatic information. In some middens individual dumps of material can be discerned and analysed.

A shell midden or shell mound is an archaeological feature consisting mainly of mollusc shells. The Danish term køkkenmøddinger (plural) was first used by Japetus Steenstrup to describe shell heaps and continues to be used by some researchers. A midden, by definition, contains the debris of human activity, and should not be confused with wind- or tide-created beach mounds. Some shell middens are processing remains: areas where aquatic resources were processed directly after harvest and prior to use or storage in a distant location.

Certain shell middens are linked directly to villages, serving as designated dump sites. In other cases, the materials found in the middens are closely tied to individual houses within the village, where each household would dispose of its waste right outside their home. Regardless of their association, shell middens are highly intricate and challenging to excavate completely and accurately. The fact that they contain a detailed record of what food was eaten or processed and many fragments of stone tools and household goods makes them invaluable objects of archaeological study.

Shells have a high calcium carbonate content, which tends to make the middens alkaline. This slows the normal rate of decay caused by soil acidity, leaving a relatively high proportion of organic material (food remnants, organic tools, clothing, human remains) available for archaeologists to find.

Edward Sylvester Morse conducted one of the first archaeological excavations of the Omori Shell Mounds in Tokyo, Japan in 1877, which led to the discovery of a style of pottery described as "cord-marked", translated as "Jōmon", which came to be used to refer to the early period of Japanese history when this style of pottery was produced. Shell middens were studied in Denmark in the latter half of the 19th century. The Danish word køkkenmødding (kitchen mound) is now used internationally. The English word "midden" (waste mound) derives from the same Old Norse word that produced the modern Danish one.

Shell middens are found in coastal or lakeshore zones all over the world. Consisting mostly of mollusc shells, they are interpreted as being the waste products of meals eaten by nomadic groups or hunting parties. Some are small examples relating to meals had by a handful of individuals, others are many metres in length and width and represent centuries of shell deposition. In Brazil, they are known as sambaquis, having been created over a long period between the 6th millennium BCE and the beginning of European colonisation.

European shell middens are primarily found along the Atlantic seaboard and in Denmark and primarily date to the 5th millennium BCE (Ertebølle and Early Funnel Beaker cultures), containing the remains of the earliest Neolithisation process (pottery, cereals and domestic animals). Younger shell middens are found in Latvia (associated with Comb Ware ceramics), Sweden (associated with Pitted Ware ceramics), the Netherlands (associated with Corded Ware ceramics) and Schleswig-Holstein (Late Neolithic and Iron Age). All these are examples where communities practised a mixed farming and hunting/gathering economy.