Æthelred was the son of King Edgar and Queen Ælfthryth. He came to the throne at about the age of 12, following the assassination of his older half-brother, Edward the Martyr. His brother's murder was carried out by supporters of his own claim to the throne, although he was too young to have any personal involvement. The chief problem of Æthelred's reign was conflict with the Danes. After several decades of relative peace, Danish raids on English territory began again in earnest in the 980s. Following the Battle of Maldon in 991, Æthelred paid tribute, or Danegeld, to the Danish king. In 1002, Æthelred ordered what became known as the St. Brice's Day massacre of Danish settlers. In 1013, King Sweyn Forkbeard of Denmark invaded England, as a result of which Æthelred fled to Normandy in 1013 and was replaced by Sweyn. However, he returned as king for two years after Sweyn's death in 1014. Æthelred's 37-year reign was the longest of any Anglo-Saxon king of England, and was only surpassed in the 13th century, by Henry III. Æthelred was briefly succeeded by his son, Edmund Ironside, but he died after a few months and was replaced by Sweyn's son, Cnut. Another of his sons, Edward the Confessor, became king in 1042.
Æthelred's first name, composed of the elements æðele, "noble", and ræd, "counsel, advice", is typical of the compound names of those who belonged to the royal House of Wessex, and it characteristically alliterates with the names of his ancestors, like Æthelwulf ("noble-wolf"), Ælfred ("elf-counsel"), Eadweard ("rich-protection"), and Eadgar ("rich-spear").
The story of Æthelred's notorious nickname, Old English Unræd, goes a long way toward explaining how his reputation has declined through history It is usually translated into present-day English as "The Unready" (less often, though less confusingly, as "The Redeless"). The Anglo-Saxon noun unræd means "evil counsel", "bad plan", or "folly". It most often describes decisions and deeds, and once refers to the nature of Satan's deceit. The element ræd in unræd is the element in Æthelred's name which means "counsel". Thus Æþelræd Unræd is a pun meaning "Noble counsel, No counsel". The nickname has alternatively been taken adjectivally as "ill-advised", "ill-prepared", "indecisive", thus "Æthelred the ill-advised".
Because the nickname was first recorded in the 1180s, more than 150 years after Æthelred's death, it is doubtful that it carries any implications for how the king was seen by his contemporaries or near contemporaries.
Sir Frank Stenton remarked that "much that has brought condemnation of historians on King Æthelred may well be due in the last resort to the circumstances under which he became king." Æthelred's father, King Edgar, had died suddenly in July 975, leaving two young sons behind. The elder, Edward (later Edward the Martyr), was probably illegitimate, and was "still a youth on the verge of manhood" in 975. The younger son was Æthelred, whose mother, Ælfthryth, Edgar had married in 964. Ælfthryth was the daughter of Ordgar, ealdorman of Devon, and widow of Æthelwold, Ealdorman of East Anglia. At the time of his father's death, Æthelred could have been no more than 10 years old. As the elder of Edgar's sons, Edward – reportedly a young man given to frequent violent outbursts – probably would have naturally succeeded to the throne of England despite his young age, had not he "offended many important persons by his intolerable violence of speech and behaviour." In any case, a number of English nobles took to opposing Edward's succession and to defending Æthelred's claim to the throne; Æthelred was, after all, the son of Edgar's last, living wife, and no rumour of illegitimacy is known to have plagued Æthelred's birth, as it might have his elder brother's. Both boys, Æthelred certainly, were too young to have played any significant part in the political manoeuvring which followed Edgar's death. It was the brothers' supporters, and not the brothers themselves, who were responsible for the turmoil which accompanied the choice of a successor to the throne. Æthelred's cause was led by his mother and included Ælfhere, Ealdorman of Mercia and Bishop Æthelwold of Winchester, while Edward's claim was supported by Dunstan, the Archbishop of Canterbury and Oswald, the Archbishop of York among other noblemen, notably Æthelwine, Ealdorman of East Anglia, and Byrhtnoth, ealdorman of Essex. In the end, Edward's supporters proved the more powerful and persuasive, and he was crowned king at Kingston upon Thames before the year was out.