The precise borders of Elmet are unclear. The term was used in medieval times as an affix to place names in the West of the old Barkston Ash and East of the old Skyrack wapentakes (between Leeds and Selby) including Burton Salmon, Sutton (east of Castleford), Micklefield, Sherburn in Elmet, Kirkby Wharfe, Saxton, Clifford and Barwick in Elmet. In the Tribal Hidage the extent of Elmet is described as 600 hides of land, an area slightly more than the total of the wapentakes of Barkston Ash and Skyrack. Some have concluded that those two wapentakes approximately represented the area of Elmet, although a hide is not a true measure of land area.
Some have argued that the kingdom of Elmet, until it was conquered in 616 or 626, was bounded by the River Sheaf in the south and the River Wharfe in the east. It adjoined Deira to the north and Mercia to the south, and its western boundary appears to have been near Craven, which was possibly a minor British kingdom. As such it was well to the east of other territories of the Britons in Wales and the West Country (i.e. Cornwall and Dumnonia), and to the south of others in the Hen Ogledd ("Old North"). As one of the southeasternmost Brittonic regions for which there is reasonably substantial evidence, it is notable for having survived relatively late in the period of Anglo-Saxon settlement of Britain.
Elmet is chiefly attested in toponymic and archaeological evidence, references in early Welsh poetry, and historical sources such as the Historia Brittonum and Bede. The Historia Brittonum provides the only direct evidence that it was a kingdom. It says that King Edwin of Northumbria "occupied Elmet and expelled Certic, king of that country". Bede's Ecclesiastical History of the English People says that Hereric, the father of Hilda of Whitby, an important figure in the Christianisation of the Anglo-Saxons, was killed at the court of Ceretic. It is generally presumed that Ceretic/Certic was the same person known in Welsh sources as Ceredig ap Gwallog, king of Elmet. However, Bede does not speak of Elmet as the name of a kingdom but rather of the silva Elmete "forest of Elmet". He mentions that "subsequent kings made a house for themselves in the district, which is called Loidis", and the battle of the Winwaed was also in the region of Loidis – probably the area covered by the present day City of Leeds.
From this evidence it appears that Elmet was one of a number of Sub-Roman Brittonic realms in the Hen Ogledd – what is now northern England and southern Scotland – during the Early Middle Ages. Other kingdoms included Rheged, the Kingdom of Strathclyde, and Gododdin. It is unclear how Elmet came to be established, though it has been suggested that it may have been created from a larger kingdom ruled by the semi-legendary Coel Hen. The historian Alex Woolf suggests that the region of Elmet had a distinct tribal identity in pre-Roman times and that this re-emerged after Roman rule collapsed.
The name of Elmet is probably Brythonic, but its origin is obscure. It is probably the same as the Welsh Elfed, the name of a cantref in Dyfed. Elmet appears to have had ties with Wales; an early Christian inscription found in Gwynedd reads "ALIOTVS ELMETIACOS HIC IACET", or "Aliotus the Elmetian lies here". A number of ancestors of Ceretic are recorded in Welsh sources: one of Taliesin's poems is for his father, Gwallog ap Lleenog, who may have ruled Elmet near the end of the 6th century.