The institution known as leiðangr (Old Norse), leidang (Norwegian), leding (Danish), ledung (Swedish), expeditio (Latin) or sometimes lething (English), was a form of conscription to organise coastal fleets for seasonal excursions and in defence of the realm typical for medieval Scandinavians and, later, a public levy of free farmers. In Anglo-Saxon England, a different system was used to achieve similar ends, and was known as the fyrd.
The leidangr of Norway is first mentioned AD 985 by Scaldic Courtly Poets of Jarl Haakon of Western Norway and his son Erik. In each poem, the princes are praised for summoning the ships of the leidangr to the Battle of Hjorungavaagr against a Danish fleet. Later on the King of Norway, Harald Hardraade (died 1066) is praised by two Court-Scalds for summoning the leidangr to attack Denmark. Harald is also called king of the leidangr and the latter is termed almenningr, the duty and right of all men. During the 11th century Danish naval forces though not termed leidangr, are sporadically praised as led by Danish kings (as Knut in his conquest of England). May 21, 1085 a Danish royal charter stipulates, that certain people on the lands of the canons of Lund are liable to pay fines for neglecting expeditio. A serious discussion concerning the source-critical value of Scaldic Poetry is ongoing between Niels Lund and Rikke Malmros.
The leiðangr was established in the medieval era, and not the Viking Age. It has been considered the maritime version of the Germanic system of hundreds which was described as early as 98 AD by Tacitus as the centeni. Since Tacitus also said the Suiones (Swedes) had a powerful fleet, it might have been based on the leidang. However, since all our sources on the leidang are medieval (the earliest, the Older Law of the Gulating, is 11th century at the absolute earliest, and might well be 12th century) this is highly uncertain. Before the establishment of the leidang, the defence of the realm was probably based on voluntary contribution to a defence-fleet. With the rise of the monarchies, the contribution became a duty.