William I, (1027-1087) also known as William the Conqueror and William of Normandy, was the illegitimate son of Robert, Duke of Normandy. When Robert died without legitimate male issue in 1035, William, despite his illegitimacy, inherited the title. William reached majority and began to govern Normandy in 1045, successfully defeating a rebellion two years later. Throughout his rule in Normandy, William continued to face threats and opposition to his right to the title.

In 1051, William visited Edward the Confessor, then king of England, claiming that Edward had promised to name William as his heir to the throne. In 1064, Harold Godwinsson (who subsequently became king of England) traveled to Normandy. William later claimed that during the visit, Harold promised he would support William's claim to the English throne.

William married Matilda of Flanders, the daughter of Count Baldwin of Flanders in 1053. They had seven surviving children.

William invaded England in September, 1066 and defeated the English at the Battle of Hastings. William was crowned king of England (William I) at Westminster Abbey. William maintained control by replacing the Anglo-Saxon nobles with Norman nobles, English clergy with French clergy, introduced French as the language of court, reduced surviving English nobles of The Battle of Hastings to serfdom and destroyed much existing Anglo-Saxon art. He maintained the Anglo-Saxon judicial system, maintained domestic slavery (9% of the English population was then enslaved), but banned the sale of slaves overseas.

William spent much time, effort, energy and resources in reorganizing England. Some historians claim that he implemented and codified feudalism in England. He was a successful administrator, and much of subsequent English law and social reform can trace its roots to William's changes.

Begun under William I, and continued under his heir, William II, was the usurpation of vast tracts of land for the king's forest. In some places, whole villages were destroyed and their inhabitants driven out. Trespassers and poachers (of both fauna and flora-trees) were subjected to drastic penalties.

Perhaps one of William's most notable legacies is the Domesday Book. At Christmas of 1085, he ordered a survey of the land. Perhaps primarily a vehicle for tax assessment, it was also important as a basis for assignment of feudal rights and duties.

William died in 1087 and was succeeded by his second son, William II, (William Rufus or William the Red).