Viking expansion: Viking raiders, possibly led by the legendary Ragnar Lodbrok, plundered and occupied Paris (depicted), holding the city for a large ransom.

Viking expansion was the historical movement which led Norse explorers, traders and warriors, the latter known in modern scholarship as Vikings, to sail most of the North Atlantic, reaching south as far as North Africa and east as far as Russia, and through the Mediterranean as far as Constantinople and the Middle East, acting as looters, traders, colonists and mercenaries. To the west, Vikings under Leif Erikson, the heir to Erik the Red, reached North America and set up a short-lived settlement in present-day L'Anse aux Meadows, Newfoundland, Canada. Longer lasting and more established Norse settlements were formed in Greenland, Iceland, the Faroe Islands, Russia, Ukraine, Great Britain, Ireland, Normandy and Sicily.

Vikings were a seafaring people originally from Scandinavia, who from the late 8th to the late 11th centuries raided, pirated, traded, and settled throughout parts of Europe. They voyaged as far as the Mediterranean, North Africa, the Middle East, Greenland, and Vinland. In their countries of origin, and in some of the countries they raided and settled, this period of activity is popularly known as the Viking Age, and the term "Viking" also commonly includes the inhabitants of the Scandinavian homelands as a whole during the late 8th to the mid-11th centuries. The Vikings had a profound impact on the early medieval history of northern and Eastern Europe, including the political and social development of England and parts of France, and established the embryo of Russia in Kievan Rus'.

Ragnar Lodbrok, according to legends, was a Viking hero and a Swedish and Danish king.

The siege of Paris of 845 was the culmination of a Viking invasion of West Francia. The Viking forces were led by a Norse chieftain named "Reginherus", or Ragnar, who tentatively has been identified with the legendary saga character Ragnar Lodbrok. Reginherus's fleet of 120 Viking ships, carrying thousands of warriors, entered the Seine in March and sailed up the river.