William the Conqueror (c. 1028 – 9 September 1087), sometimes called William the Bastard, was the first Norman king of England (as William I), reigning from 1066 until his death. A descendant of Rollo, he was Duke of Normandy (as William II) from 1035 onward. By 1060, following a long struggle, his hold on Normandy was secure. In 1066, following the death of Edward the Confessor, William invaded England, leading a Franco-Norman army to victory over the Anglo-Saxon forces of Harold Godwinson at the Battle of Hastings, and suppressed subsequent English revolts in what has become known as the Norman Conquest. The rest of his life was marked by struggles to consolidate his hold over England and his continental lands, and by difficulties with his eldest son, Robert Curthose.

William was the son of the unmarried Duke Robert I of Normandy and his mistress Herleva. His illegitimate status and youth caused some difficulties for him after he succeeded his father, as did the anarchy which plagued the first years of his rule. During his childhood and adolescence, members of the Norman aristocracy battled each other, both for control of the child duke, and for their own ends. In 1047, William quashed a rebellion and began to establish his authority over the duchy, a process that was not complete until about 1060. His marriage in the 1050s to Matilda of Flanders provided him with a powerful ally in the neighbouring county of Flanders. By the time of his marriage, William was able to arrange the appointment of his supporters as bishops and abbots in the Norman church. His consolidation of power allowed him to expand his horizons, and he secured control of the neighbouring county of Maine by 1062.

In the 1050s and early 1060s, William became a contender for the throne of England held by the childless Edward the Confessor, his first cousin once removed. There were other potential claimants, including the powerful English earl Harold Godwinson, whom Edward named as king on his deathbed in January 1066. Arguing that Edward had previously promised the throne to him and that Harold had sworn to support his claim, William built a large fleet and invaded England in September 1066. He decisively defeated and killed Harold at the Battle of Hastings on 14 October 1066. After further military efforts, William was crowned king on Christmas Day, 1066, in London. He made arrangements for the governance of England in early 1067 before returning to Normandy. Several unsuccessful rebellions followed, but William's hold on England was mostly secure by 1075, allowing him to spend the greater part of his reign in continental Europe.

William's final years were marked by difficulties in his continental domains, troubles with his son, Robert, and threatened invasions of England by the Danes. In 1086, he ordered the compilation of the Domesday Book, a survey listing all of the land-holdings in England along with their pre-Conquest and current holders. He died in September 1087 while leading a campaign in northern France, and was buried in Caen. His reign in England was marked by the construction of castles, settling a new Norman nobility on the land, and change in the composition of the English clergy. He did not try to integrate his domains into one empire but continued to administer each part separately. His lands were divided after his death: Normandy went to Robert, and England went to his second surviving son, William Rufus.

Norsemen first began raiding in what became Normandy in the late 8th century. Permanent Scandinavian settlement occurred before 911, when Rollo, one of the Viking leaders, and King Charles the Simple of France reached an agreement ceding the county of Rouen to Rollo. The lands around Rouen became the core of the later duchy of Normandy. Normandy may have been used as a base when Scandinavian attacks on England were renewed at the end of the 10th century, which would have worsened relations between England and Normandy. In an effort to improve matters, King Æthelred the Unready took Emma, sister of Richard II, Duke of Normandy, as his second wife in 1002.

Danish raids on England continued, and Æthelred sought help from Richard, taking refuge in Normandy in 1013 when King Swein I of Denmark drove Æthelred and his family from England. Swein's death in 1014 allowed Æthelred to return home, but Swein's son Cnut contested Æthelred's return. Æthelred died unexpectedly in 1016, and Cnut became king of England. Æthelred and Emma's two sons, Edward and Alfred, went into exile in Normandy while their mother, Emma, became Cnut's second wife.

After Cnut's death in 1035, the English throne fell to Harold Harefoot, his son by his first wife, while Harthacnut, his son by Emma, became king in Denmark. England remained unstable. Alfred returned to England in 1036 to visit his mother and perhaps to challenge Harold as king. One story implicates Earl Godwin of Wessex in Alfred's subsequent death, but others blame Harold. Emma went into exile in Flanders until Harthacnut became king following Harold's death in 1040, and his half-brother Edward followed Harthacnut to England; Edward was proclaimed king after Harthacnut's death in June 1042.

William was born in 1027 or 1028 at Falaise, Duchy of Normandy, most likely towards the end of 1028. He was the only son of Robert I, son of Richard II. His mother, Herleva, was a daughter of Fulbert of Falaise, who may have been a tanner or embalmer. Herleva was possibly a member of the ducal household, but did not marry Robert. She later married Herluin de Conteville, with whom she had two sons – Odo of Bayeux and Count Robert of Mortain – and a daughter whose name is unknown. One of Herleva's brothers, Walter, became a supporter and protector of William during his minority. Robert I also had a daughter, Adelaide, by another mistress.