Notgeld (German for 'emergency money' or 'necessity money') is money issued by an institution in a time of economic or political crisis. The issuing institution is usually one without official sanction from the central government. This usually occurs when not enough state-produced money is available from the central bank. In particular, notgeld generally refers to money produced in Germany and Austria during World War I and the Interwar period. Issuing institutions could be a town's savings banks, municipalities and private or state-owned firms. Nearly all issues contained an expiry date, after which time they were invalid. Issues without dates ordinarily had an expiry announced in a newspaper or at the place of issuance.

Notgeld was mainly issued in the form of (paper) banknotes. Sometimes other forms were also used: coins, leather, silk, linen, wood, postage stamps, aluminium foil, coal, and porcelain; there are also reports of elemental sulfur being used, as well as all sorts of re-used paper and carton material (e.g. playing cards). These pieces made from playing cards are extremely rare and are known as Spielkarten, the German word for 'playing cards'.

Notgeld was a mutually-accepted means of payment in a particular region or locality, but notes could travel widely. Some cases of Notgeld could better be defined as scrip, which were essentially coupons redeemable only at specific businesses. However, the immense volume of issues produced by innumerable municipalities, firms, businesses, and individuals across Germany blurred the definition. Collectors tend to categorize by region or era rather than issuing authority (see below). Notgeld is different from occupation money (e.g. Japanese invasion money) that is issued by an occupying army during a war.

Arnold Keller, historian and orientalist, classified German Notgeld into different periods. Keller edited a magazine called Das Notgeld during the "collector phase" of Notgeld issuance. He compiled a series of catalogs in the years afterward. Although incomplete in many cases, his work formed the foundations of the hobby.

Notgeld was released even before Germany entered World War I. On 31 July 1914 three notes were issued by the Bürgerliches Brauhaus GmbH of Bremen (a brewery). This was due to hoarding of coins by the population in the days before war was declared. The first period of Notgeld continued until the end of 1914, but mostly ceased once the German Reichsbank made up for the shortage with issues of small denomination paper notes and coins of cheaper metal.

As the war dragged on, acute monetary shortages could not be met by the German central bank, leading to a new period of Notgeld beginning in 1916. Additionally, the non-precious metals used to mint lower value coins were needed to produce war supplies. Dr Keller arranged this period into two catalogs: Kleingeldscheine for issues of less than 1 Mark face value and Grossgeldscheine for values 1 Mark and higher. This period of issue came to a close in 1919.

Although camp money used by prisoners of war was different from Notgeld, collectors inevitably lumped this material into the hobby. The period covered the entire war, 1914–1918. This field of collecting may include World War II issues, though this covers only notes circulated in concentration camps, as the German Luftwaffe in charge of prisoners of war prepared a general issue of notes for all camps under their direction.

Though the production of Notgeld was initially amateurish, with many set by typewriter or even handwritten, collectors soon appeared on the scene to take hold of the expired 1914 stock. With the next wave of issues in the latter half of the war, Notgeld production was handled by professional printers. These issues incorporated pleasing designs, and a new reason for hoarding came into being.