Early years
William was the elder of two children of Robert I of Normandy and his concubine Herleva or Arlette, the daughter of a burgher from the town of Falaise.  In 1035 Robert died when returning from a pilgrimage to Jerusalem and William, his only son, whom he had nominated as his heir before his departure, was accepted as duke by the Norman magnates and his feudal overlord, King Henry I of France.  William and his friends had to overcome enormous obstacles.  His illegitimacy (he was generally known as the Bastard) was a handicap, and he had to survive the collapse of law and order that accompanied his accession as a child.

Three of William's guardians died violent deaths before he grew up, and his tutor was murdered.  His father's kin were of little help; most of them thought that they stood to gain by the boy's death.  But his mother managed to protect William through the most dangerous period.  These early difficulties probably contributed to his strength of purpose and his dislike of lawlessness and misrule.

Ruler of Normandy
By 1042, when William reached his 15th year, was knighted and began to play a personal part in the affairs of his duchy, the worst was over.  But his attempts to recover rights lost during the anarchy and to bring disobedient vassals and servants to heel inevitably led to trouble.  From 1046 until 1055 he dealt with a series of baronial rebellions, mostly led by kinsmen.   Occasionally he was in great danger and had to rely on Henry of France for help.  In 1047 Henry and William defeated a coalition of Norman rebels at Val-ès-Dunes, southeast of Caen.  It was in these years that William learned to fight and rule.

William soon learned to control his youthful recklessness.  He was always ready to take calculated risks on campaign and, most important, to fight a battle.  But he was not a chivalrous or flamboyant commander.  His plans were simple, his methods direct, and he exploited ruthlessly any advantage gained.  If he found himself at a disadvantage, he withdrew immediately.  He showed the same qualities in his government.  He never lost sight of his aim to recover lost ducal rights and revenues, and, although he developed no theory of government or great interest in administrative techniques, he was always prepared to improvise and experiment.  He seems to have lived a moral life by the standards of the time, and he acquired an interest in the welfare of the Norman church.  He made his half brother, Odo, bishop of Bayeux in 1049 at the age of about 16, and Odo managed to combine the roles of nobleman and prelate in a way that did not greatly shock contemporaries.  But William also welcomed foreign monks and scholars to Normandy.  Lanfranc of Pavia, a famous master of the liberal arts, who entered the monastery of Bec about 1042, was made abbot of Caen in 1063.

According to a brief description of William's person by an anonymous author, who borrowed extensively from Einhard's Life of Charlemagne, he was just above average height and had a robust, thick-set body.  Though he was always sparing of food and drink, he became fat in later life.  He had a rough bass voice and was a good and ready speaker.  Writers of the next generation agree that he was exceptionally strong and vigorous.  William was an out-of-doors man, a hunter and soldier, fierce and despotic, generally feared; uneducated, he had few graces but was intelligent and shrewd and soon obtained the respect of his rivals.

New alliances
After 1047 William began to take part in events outside his duchy.  In support of his lord, King Henry, and in pursuit of an ambition to strengthen his southern frontier and expand into Maine, he fought a series of campaigns against Geoffrey Martel, count of Anjou.  But in 1052 Henry and Geoffrey made peace, there was a serious rebellion in eastern Normandy, and, until 1054 William was again in serious danger.  During this period he conducted important negotiations with his cousin Edward the Confessor, king of England, and took a wife.

Norman interest in Anglo-Saxon England derived from an alliance made in 1002, when King Ethelred II of England married Emma, the sister of Count Richard II, William's grandfather.  Two of her sons, William's cousins once removed, had reigned in turn in England: Hardecanute (1040-42) and Edward the Confessor (1042-66).  William had met Edward during that prince's exile on the Continent and may well have given him some support when he returned to England in 1041.  In that year Edward was about 36 and William 14.  It is clear that William expected some sort of reward from Edward and, when Edward's marriage proved unfruitful, began to develop an ambition to become his kinsman's heir.  Edward probably at times encouraged William's hopes.  His childlessness was a diplomatic asset.

In 1049 William negotiated with Baldwin V of Flanders for the hand of his daughter, Matilda.  Baldwin, an imperial vassal with a distinguished lineage, was in rebellion against the Western emperor, Henry III, and in desperate need of allies.  The proposed marriage was condemned as incestuous (William and Matilda were evidently related in some way) by the Emperor's friend, Pope Leo IX, at the Council of Reims in October 1049; but so anxious were the parties for the alliance that before the end of 1053, possibly in 1052, the wedding took place.  In 1059 William was reconciled to the papacy, and as penance the disobedient pair built two monasteries at Caen.  Four sons were born to William and Matilda: Robert (the future duke of Normandy), Richard (who died young), William Rufus (the Conqueror's successor in England), and Henry (Rufus' successor).  Among the daughters was Adela, who was the mother of Stephen, king of England.

Edward the Confessor was supporting the Emperor, and it is possible that William used his new alliance with Flanders to put pressure on Edward and extort an acknowledgment that he was the English king's heir.  At all events, Edward seems to have made some sort of promise to William in 1051, while Tostig, son of the greatest nobleman in England, Earl Godwine, married Baldwin's half sister.  The immediate purpose of this tripartite alliance was to improve the security of each of the parties.  If William secured a declaration that he was Edward's heir, he was also looking very far ahead.

Between 1054 and 1060 William held his own against an alliance between King Henry I and Geoffrey Martel of Anjou.  Both men died in 1060 and were succeeded by weaker rulers.  As a result, in 1063 William was able to conquer Maine.  In 1064 or 1065 Edward sent his brother-in-law, Harold, earl of Wessex, Godwine's son and successor, on an embassy to Normandy.  William took him on a campaign into Brittany, and in connection with this Harold swore an oath in which, according to Norman writers, he renewed Edward's bequest of the throne to William and promised to support it.

When Edward died childless on 5 January 1066, Harold was accepted as king by the English magnates, and William decided on war.  Others, however, moved more quickly.  In May Tostig, Harold's exiled brother, raided England, and in September he joined the invasion forces of Harald III Hardraade, king of Norway, off the Northumbrian coast.  William assembled a fleet, recruited an army, and gathered his forces in August at the mouth of the Dives River.  It is likely that he originally intended to sail due north and invade England by way of the Isle of Wight and Southampton Water.  Such a plan would give him an offshore base and interior lines.  But adverse winds detained his fleet in harbour for a month, and in September a westerly gale drove his ships up-Channel.

The Battle of Hastings
William regrouped his forces at Saint-Valéry on the Somme.  He had suffered a costly delay, some naval losses, and a drop in the morale of his troops.  On September 27, after cold and rainy weather, the wind backed south.  William embarked his army and set sail for the southeast coast of England.  The following morning he landed, took the unresisting towns of Pevensey and Hastings, and began to organize a bridgehead with between 4,000 and 7,000 cavalry and infantry.

William's forces were in a narrow coastal strip, hemmed in by the great forest of Andred, and, although this corridor was easily defensible, it was not much of a base for the conquest of England.  The campaigning season was almost past, and when William received news of his opponent it was not reassuring.  On 25 September Harold had defeated and slain Tostig and Harald Hardraade at Stamford Bridge, near York, and was retracing his steps to meet the new invader.  On 13 October, when Harold emerged from the forest, William was taken by surprise.  But the hour was too late for Harold to push on to Hastings, and he took up a defensive position.  Early the next day William went out to give battle.  He attacked the English phalanx with archers and cavalry but saw his army almost driven from the field.  He rallied the fugitives, however, and brought them back into the fight and in the end wore down his opponents.  Harold's brothers were killed early in the battle.  Toward nightfall the King himself fell and the English gave up.  William's coolness and tenacity secured him victory in this fateful battle, and he then moved against possible centres of resistance so quickly that he prevented a new leader from emerging.  On Christmas Day 1066 he was crowned king in Westminster Abbey.  In a formal sense the Norman Conquest of England had taken place.

King of England
William was already an experienced ruler.  In Normandy he had replaced disloyal nobles and ducal servants with his own friends, limited private warfare, and recovered usurped ducal rights, defining the feudal duties of his vassals.  The Norman church flourished under his rule.  He wanted a church free of corruption but subordinate to him.  He would not tolerate opposition from bishops and abbots or interference from the papacy.  He presided over church synods and reinforced ecclesiastical discipline with his own.  In supporting Lanfranc, prior of Bec, against Berengar of Tours in their dispute over the doctrine of the Eucharist, he found himself on the side of orthodoxy.  He was never guilty of the selling of church office (simony).  He disapproved of clerical marriage.  At the same time he was a stern and sometimes rough master, swayed by political necessities, and he was not generous to the church with his own property.  The reformer Lanfranc was one of his advisers; but perhaps even more to his taste were the worldly and soldierly bishops Odo of Bayeux and Geoffrey of Coutances.

William left England early in 1067 but had to return in December because of English unrest.  The English rebellions that began in 1067 reached their peak in 1069 and were finally quelled in 1071.  They completed the ruin of the highest English aristocracy and gave William a distaste for his newly conquered kingdom.  Since his position on the Continent was deteriorating, he wanted to solve English problems as cheaply as possible.  To secure England's frontiers, he invaded Scotland in 1072 and Wales in 1081 and created special defensive "marcher" counties along the Scottish and Welsh borders.

In the last 15 years of his life he was more often in Normandy than in England, and there were five years, possibly seven, in which he did not visit the kingdom at all.  He retained most of the greatest Anglo-Norman barons with him in Normandy and confided the government of England to bishops, trusting especially his old friend Lanfranc, whom he made archbishop of Canterbury.  Much concerned that the natives should not be unnecessarily disturbed, he allowed them to retain their own laws and courts.

William returned to England only when it was absolutely necessary: in 1075 to deal with the aftermath of a rebellion by Roger, earl of Hereford, and Ralf, earl of Norfolk, which was made more dangerous by the intervention of a Danish fleet; and in 1082 to arrest and imprison his half brother Odo, bishop of Bayeux and earl of Kent, who was planning to take an army to Italy, perhaps to make himself pope.  In the spring of 1082 William had his son Henry knighted, and in August at Salisbury he took oaths of fealty from all the important landowners in England, whosoever's vassals they might be.  In 1085 he returned with a large army to meet the threat of an invasion by Canute IV (Canute the Holy) of Denmark.  When this came to nothing owing to Canute's death in 1086, William ordered an economic and tenurial survey to be made of the kingdom, the results of which are summarised in the two volumes of Domesday Book.

William was preoccupied with the frontiers of Normandy.  The danger spots were in Maine and the Vexin on the Seine, where Normandy bordered on the French royal demesne.  After 1066 William's continental neighbours became more powerful and even more hostile.  In 1068 Fulk the Surly succeeded to Anjou and in 1071 Robert the Frisian to Flanders.  Philip I of France allied with Robert and Robert with the Danish king, Canute IV.  There was also the problem of William's heir apparent, Robert Curthose, who, given no appanage and seemingly kept short of money, left Normandy in 1077 and intrigued with his father's enemies.  In 1081 William made a compromise with Fulk in the treaty of Blancheland: Robert Curthose was to be count of Maine but as a vassal of the count of Anjou.  The eastern part of the Vexin, the county of Mantes, had fallen completely into King Philip's hands in 1077 when William had been busy with Maine.  In 1087 William demanded from Philip the return of the towns of Chaumont, Mantes, and Pontoise.  In July he entered Mantes by surprise, but while the town burned he suffered some injury from which he never recovered.  He was thwarted at the very moment when he seemed about to enforce his last outstanding territorial claim.

Death
William was taken to a suburb of Rouen, where he lay dying for five weeks.  He had the assistance of some of his bishops and doctors, and in attendance were his half brother Robert, count of Mortain, and his younger sons, William Rufus and Henry.  Robert Curthose was with the King of France.  It had probably been his intention that Robert, as was the custom, should succeed to the whole inheritance.  In the circumstances he was tempted to make the loyal Rufus his sole heir.  In the end he compromised: Normandy and Maine went to Robert and England to Rufus.  Henry was given great treasure with which he could purchase an appanage.  William died at daybreak on 9 September, in his 60th year, and was buried in rather unseemly fashion in St Stephen's Church, which he had built at Caen.