Scholars disagree about the reasons for Viking attacks and migrations. A very unstable Danish kingship and disputes over the succession led to civil war and disorder, which may have driven warriors abroad in search of booty and supporters. The population of Scandinavia may have grown too large for the available land to support, and cities on the coasts of northern Europe offered targets for plunder. Goods plundered could then be sold, and looting raids turned into trading ventures. Some scholars assert that the Vikings were looking for trade and new commercial contacts from the beginning.

Whatever the motivations, Viking attacks were savage. Propelled either by oars or by sails, deckless, and about sixty-five feet long, a Viking ship could carry between forty and sixty men — enough to harass an isolated monastery or village. The Vikings burned, looted, and did extensive property damage, although there is little evidence that they caused long-term physical destruction — perhaps because, arriving in small bands, they lacked the manpower to do so. They seized magnates and high churchmen and held them for ransom; they also demanded tribute from kings.

The slave trade represented an important part of Viking plunder and commerce. Slaves, known as thralls, were common in Scandinavian society, and Vikings took people from the British Isles and territories along the Baltic Sea as part of their booty. They sold them as slaves in the markets of Magdeburg and Regensburg, at the fairs of Lyons, and in seaports of the Muslim world. Dublin became a center of the Viking slave trade, with hundreds and sometimes thousands of young men and women bought and sold there in any one year.

In the early tenth century, Danish Vikings besieged Paris with fleets of more than a hundred highly maneuverable ships, and the Frankish king Charles the Simple bought them off in 911 by giving them a large part of northern France. There the Vikings established the province of “Northmanland,” or Normandy as it was later known, intermarrying with the local population and creating a distinctive Norman culture. From there they sailed around Spain and into the Mediterranean, eventually seizing Sicily from the Muslim Arabs in 1060–1090, while other Normans crossed the English Channel, defeating Anglo-Saxon forces in 1066. Between 850 and 1000, Viking control of northern Europe reached its zenith. Norwegian Vikings moved farther west than any Europeans had before, establishing permanent settlements on Iceland and short-lived settlements in Greenland and Newfoundland in what is now Canada.

The Vikings made positive contributions to the areas they settled. They carried their unrivaled knowledge of shipbuilding and seamanship everywhere. The northeastern and central parts of England where the Vikings settled became known as the Danelaw because Danish, not English, laws and customs prevailed there. Scholars believe that some legal institutions, such as the ancestor of the modern grand jury, originated in the Danelaw. Exports from Ireland included iron tools and weapons manufactured there by Viking metal-smiths.