the military conquest of England by William (William I), duke of Normandy, primarily effected by his decisive victory at the Battle of Hastings (Hastings, Battle of) (Oct. 14, 1066) and resulting ultimately in profound political, administrative, and social changes in the British Isles.

The conquest was the final act of a complicated drama that had begun years earlier, in the reign of Edward the Confessor, last king of the Anglo-Saxon royal line. Edward, who had almost certainly designated William as his successor in 1051, was involved in a childless marriage and used his lack of an heir as a diplomatic tool, promising the throne to different parties throughout his reign, including Harold Godwineson, later Harold II, the powerful earl of Wessex. The exiled Tostig (Northumbria, Tostig, earl of), who was Harold's brother, and Harald III Hardraade (Harald III Sigurdsson), king of Norway, also had designs on the throne and threatened invasion. Amid this welter of conflicting claims, Edward from his deathbed named Harold his successor on Jan. 5, 1066, and Harold was crowned king the following day. However, Harold's position was compromised, according to the Bayeux Tapestry and other Norman sources, because in 1064 he had sworn an oath, in William's presence, to defend William's right to the throne.

From almost the beginning of his reign, Harold (Harold II) faced challenges to his authority. Tostig began raiding the southern and eastern coasts of England in May, eventually joining forces with Harald III. Harold was able to keep his militia on guard throughout the summer but dismissed it early in September, when he ran out of supplies and his peasant soldiers needed to return to their fields for the harvest. This left the south without defenses, exposing it to invasion by William. Before William arrived, however, Harald III and Tostig invaded in the north; Harold hastened to Yorkshire, where at Stamford Bridge (September 25) he won a smashing victory in which both Harald III and Tostig perished.

Harold's wall of highly trained infantry held firm in the face of William's mounted assault; failing to breach the English lines and panicked by the rumour of William's death, the Norman cavalry fled in disorder. But William, removing his helmet to show he was alive, rallied his troops, who turned and killed many English soldiers. As the battle continued, the English were gradually worn down; late in the afternoon, Harold was killed (by an arrow in the eye, according to the Bayeux Tapestry), and by nightfall the remaining English had scattered and fled. William then made a sweeping advance to isolate London, and at Berkhamstead the major English leaders submitted to him. He was crowned in Westminster Abbey on Christmas Day, 1066. Sporadic indigenous revolts continued until 1071; the most serious, in Northumbria (1069–70), was suppressed by William himself, who then devastated vast tracts of the north. The subjection of the country was completed by the rapid building of a great number of castles.

The extent and desirability of the changes brought about by the conquest have long been disputed by historians. Certainly, in political terms, William's victory destroyed England's links with Scandinavia, bringing the country instead into close contact with the Continent, especially France. Inside England the most radical change was the introduction of land tenure (feudal land tenure) and military service. While tenure of land in return for services had existed in England before the conquest, William revolutionized the upper ranks of English society by dividing the country among about 180 Norman tenants-in-chief and innumerable mesne (intermediate) tenants, all holding their fiefs by knight service. The result, the almost total replacement of the English aristocracy with a Norman one, was paralleled by similar changes of personnel among the upper clergy and administrative officers.

Anglo-Saxon England had developed a highly organized central and local government and an effective judicial system (see Anglo-Saxon law). All these were retained and utilized by William, whose coronation oath showed his intention of continuing in the English royal tradition. The old administrative divisions were not superseded by the new fiefs, nor did feudal justice normally usurp the customary jurisdiction of shire and hundred courts. In them and in the king's court, the common law of England continued to be administered. Innovations included the new but restricted body of “forest law” and the introduction in criminal cases of the Norman trial by combat alongside the old Saxon ordeals. Increasing use was made of the inquest procedure—the sworn testimony of neighbours, both for administrative purposes and in judicial cases. A major change was William's removal of ecclesiastical cases from the secular courts, which allowed the subsequent introduction into England of the then rapidly growing canon law.

William also transformed the structure and character of the church in England. He replaced all the Anglo-Saxon bishops, except Wulfstan of Dorchester, with Norman bishops. Most notably, he secured the deposition of Stigand, the archbishop of Canterbury—who held his see irregularly and had probably been excommunicated by Pope Leo IX (Leo IX, Saint)—and appointed in his place Lanfranc of Bec, a respected scholar and one of William's close advisers. Seeking to impose a more orderly structure on the English episcopacy, the king supported Lanfranc's claims for the primacy of Canterbury in the English church. William also presided over a number of church councils, which were held far more frequently than under his predecessors, and introduced legislation against simony (the selling of clerical offices) and clerical marriage. A supporter of monastic reform while duke of Normandy, William introduced the latest reforming trends to England by replacing Anglo-Saxon abbots with Norman ones and by importing numerous monks. Although he founded only a small number of monasteries, including Battle Abbey (in honour of his victory at Hastings), William's other measures contributed to the quickening of monastic life in England.

Probably the most regrettable effect of the conquest was the total eclipse of the English vernacular as the language of literature, law, and administration. Superseded in official documents and other records by Latin and then increasingly in all areas by Anglo-Norman, written English hardly reappeared until the 13th century.