The institution known as leiðangr (Old Norse), leidang (Norwegian), leding, (Danish), ledung (Swedish), expeditio (Latin) or sometimes lething (in English language), was a public fleet levy of free farmers typical for the Viking Age Scandinavians. The leiðangr was however established long before the beginning of the Viking Age. It was the maritime version of the Germanic system of hundreds which was described as early as 98 A.D. by Tacitus as the centeni. Since Tacitus also said that the Swedes (Suiones) had a powerful fleet, it seems likely that it was based on the leidang.

In all the three Scandinavian countries leiðangr evolved to a tax in the 12th century, paid by all (free) farmers until the 19th century. As the clergy and the noble estate became exempted from this tax burden, the tax came to facilitate the Church's and the noble manor's acquisition of land. In the end, the states became increasingly dependent on the nobility for defence and warfare.

The leiðangr was a system organising a coastal fleet with the aim of defence, coerced trade, plunderings and aggressive wars. Normally, the fleet levy was on expeditions for two or three summer months. All free men, i.e. the peasants, were obliged to take part in or contribute to the leiðangr. All of the leiðangr was called to arms when invading forces threatened the land. In the expeditions only a faction of the ships were taking part, but as the expeditions often were profitable many magnates and chieftains tried to join with their people as often as possible. In the 11th century the leiðangr was very successful. Sections of the Danish and Norwegian leiðangr conquered Western England under Canute the Great, mainly a re-conquest of the Danelaw.

The lands were divided into districts, ship's crews, "skipreiða" (Old Norse), "skipæn" (Danish) or "roslag" (Swedish). The farmers of the district had to build and equip a rowed sailing ship. The size of the ships was defined as a standardized number of oars, initially forty oars, later the standardized size of 24 was increased. In Norway, there were 279 such districts in 1277, in Denmark two-three times as many. The head of a district was called "styrimaðr" or "styræsmand", steersman, and he functioned as captain of the ship. The smallest unit was the crew of peasants who had to arm and provide for one oarsman ("hafnæ" in Danish, "manngerð" in Old Norse).

According to the Law of Uppland, the hundreds of Uppland provided as many as four ships each, those of Västmanland two ships and those of Roslagen one ship.

The weapons the ships would carry were typically: bows, spears, swords, battle- axes and shields. In the 11th century, jarls are mentioned as the chieftain of the leiðangr, in the 12th century the bishop was head of the fleet levy.

See also

• History of Norway
• History of Denmark
• History of Sweden