quality:corruption, structural:expansion
Family

Alfred was a son of Æthelwulf, king of Wessex, and his wife Osburh. According to his biographer, Asser, writing in 893, "In the year of our Lord's Incarnation 849 Alfred, King of the Anglo-Saxons", was born at the royal estate called Wantage, in the district known as Berkshire ("which is so called from Berroc Wood, where the box tree grows very abundantly"). This date has been accepted by the editors of Asser's biography, Simon Keynes and Michael Lapidge, and by other historians such as David Dumville, Justin Pollard and Richard Huscroft. West Saxon genealogical lists state that Alfred was 23 when he became king in April 871, implying that he was born between April 847 and April 848. This dating is adopted in the biography of Alfred by Alfred Smyth, who regards Asser's biography as fraudulent, an allegation which is rejected by other historians. Richard Abels in his biography discusses both sources but does not decide between them and dates Alfred's birth as 847/849, while Patrick Wormald in his Oxford Dictionary of National Biography article dates it 848/849. Berkshire had been historically disputed between Wessex and the midland kingdom of Mercia, and as late as 844, a charter showed that it was part of Mercia, but Alfred's birth in the county is evidence that, by the late 840s, control had passed to Wessex.

He was the youngest of six children. His eldest brother, Æthelstan, was old enough to be appointed sub-king of Kent in 839, almost 10 years before Alfred was born. He died in the early 850s. Alfred's next three brothers were successively kings of Wessex. Æthelbald (858–860) and Æthelberht (860–865) were also much older than Alfred, but Æthelred (865–871) was only a year or two older. Alfred's only known sister, Æthelswith, married Burgred, king of Mercia in 853. Most historians think that Osburh was the mother of all Æthelwulf's children, but some suggest that the older ones were born to an unrecorded first wife. Osburh was descended from the rulers of the Isle of Wight. She was described by Alfred's biographer Asser as "a most religious woman, noble by temperament and noble by birth". She had died by 856 when Æthelwulf married Judith, daughter of Charles the Bald, king of West Francia.

In 868, Alfred married Ealhswith, daughter of the Mercian nobleman Æthelred Mucel, ealdorman of the Gaini, and his wife Eadburh, who was of royal Mercian descent. Their children were Æthelflæd, who married Æthelred, Lord of the Mercians; Edward the Elder, Alfred's successor as king; Æthelgifu, abbess of Shaftesbury; Ælfthryth, who married Baldwin, count of Flanders; and Æthelweard.

In 868, Alfred married Ealhswith, daughter of a Mercian nobleman, Æthelred Mucel, Ealdorman of the Gaini. The Gaini were probably one of the tribal groups of the Mercians. Ealhswith's mother, Eadburh, was a member of the Mercian royal family.

They had five or six children together, including Edward the Elder who succeeded his father as king; Æthelflæd who became lady of the Mercians; and Ælfthryth who married Baldwin II, Count of Flanders. Alfred's mother was Osburga, daughter of Oslac of the Isle of Wight, Chief Butler of England. Asser, in his Vita Ælfredi asserts that this shows his lineage from the Jutes of the Isle of Wight.

Osferth was described as a relative in King Alfred's will and he attested charters in a high position until 934. A charter of King Edward's reign described him as the king's brother – mistakenly according to Keynes and Lapidge, but in the view of Janet Nelson, he probably was an illegitimate son of King Alfred.