The eighth-century monk Bede, in his Ecclesiastical History of the English People, lists him as the third king to hold imperium over other Anglo-Saxon kingdoms.

He married Bertha, the Christian daughter of Charibert I, king of the Franks, thus building an alliance with the most powerful state in contemporary Western Europe; the marriage probably took place before he came to the throne.

These groups captured territory in the east and south of England, but at about the end of the fifth century, a British victory at the battle of Mount Badon (Mons Badonicus) halted the Anglo-Saxon advance for fifty years.

[6] Some historians now think the underlying story of a rebelling mercenary force may be accurate; most now date the founding of the kingdom of Kent to the middle of the fifth-century, which is consistent with the legend.

[9] Overlordship was a central feature of Anglo-Saxon politics which began before Æthelberht's time; kings were described as overlords as late as the ninth century.

The Anglo-Saxon Chronicle, a collection of annals assembled c. 890 in the kingdom of Wessex, mentions several events in Kent during Æthelberht's reign.

[16] One other member of Æthelberht's family is known: his sister, Ricole, who is recorded by both Bede and the Anglo-Saxon Chronicle as the mother of Sæberht, king of the East Saxons (i.e., Essex).

If this does not simply reflect Gregory's ignorance of Kentish affairs, which seems unlikely given the close ties between Kent and the Franks, then some assert that Æthelberht's reign cannot have begun before 589.

This evidence is less clear for the earlier period, but there are early charters, known to be forged, which nevertheless imply that Æthelberht ruled as joint king with his son, Eadbald.

An unusual feature of the Kentish system was that only sons of kings appeared to be legitimate claimants to the throne, although this did not eliminate all strife over the succession.

There is no record that Æthelberht ever accepted a continental king as his overlord and, as a result, historians are divided on the true nature of the relationship.

The letter concerned Augustine's mission to Kent in 597, and in it Gregory says that he believes "that you wish your subjects in every respect to be converted to that faith in which you, their kings and lords, stand".

[23][24] A possible reason for the willingness of the Franks to connect themselves with the Kentish court is the fact that a Frankish king, Chilperic I, is recorded as having conquered a people known as the Euthiones during the mid-sixth century.

There was a luxury trade between Kent and the Franks, and burial artefacts found include clothing, drink, and weapons that reflect Frankish cultural influence.

The Kentish burials have a greater range of imported goods than those of the neighbouring Anglo-Saxon regions, which is not surprising given Kent's easier access to trade across the English Channel.

[26] The exact meaning of bretwalda has been the subject of much debate; it has been described as a term "of encomiastic poetry",[27] but there also is evidence that it implied a definite role of military leadership.

[28] The prior bretwalda, Ceawlin, is recorded by the Anglo-Saxon Chronicle as having fought Æthelberht in 568 at a place called "Wibbandun" ("Wibba's Mount") whose location has not been identified.

[17][33][34] Bede describes Æthelberht's relationship with Rædwald, king of East Anglia, in a passage that is not completely clear in meaning.

[36] There is no evidence that Æthelberht's influence in other kingdoms was enough for him to convert any other kings to Christianity, although this is partly due to the lack of sources—nothing is known of Sussex's history, for example, for almost all of the seventh and eighth centuries.

[14] According to Bede, Æthelberht was sufficiently distrustful of the newcomers to insist on meeting them under the open sky, to prevent them from performing sorcery.

[39] Rædwald, king of East Anglia, was only partly converted (apparently while at Æthelberht's court) and retained a pagan shrine next to the new Christian altar.

[44][45] These laws are by far the earliest surviving code composed in any of the Germanic countries,[23] and they were almost certainly among the first documents written down in Anglo-Saxon, as literacy would have arrived in England with Augustine's mission.

[46] The only surviving early manuscript, the Textus Roffensis, dates from the twelfth century, and it now resides in the Medway Studies Centre in Strood, Kent.

[47] Æthelberht's code makes reference to the church in the very first item, which enumerates the compensation required for the property of a bishop, a deacon, a priest, and so on;[45] but overall, the laws seem remarkably uninfluenced by Christian principles.

In subject matter, the laws have been compared to the Lex Salica of the Franks, but it is not thought that Æthelberht based his new code on any specific previous model.

[23][44] The laws are concerned with setting and enforcing the penalties for transgressions at all levels of society; the severity of the fine depended on the social rank of the victim.

[49] One of Æthelberht's laws seems to preserve a trace of a very old custom: the third item in the code states that "If the king is drinking at a man's home, and anyone commits any evil deed there, he is to pay twofold compensation.

[53] Æthelberht died on 24 February 616 and was succeeded by his son, Eadbald, who was not a Christian—Bede says he had been converted but went back to his pagan faith,[44] although he ultimately did become a Christian king.

There is no agreement among modern scholars on how to interpret this: "Aduluald" might be intended as a representation of "Æthelwald", and hence an indication of another king, perhaps a sub-king of west Kent;[56] or it may be merely a scribal error which should be read as referring to Eadbald.

[58] In the 2004 edition of the Roman Martyrology, he is listed under his date of death, 24 February, with the citation: 'King of Kent, converted by St Augustine, bishop, the first leader of the English people to do so'.