The Kingdom of the Isles comprised the Hebrides, the islands of the Firth of Clyde and the Isle of Man from the 9th to the 13th centuries AD. The islands were known to the Norse as the Suðreyjar, or "Southern Isles" as distinct from the Norðreyjar or Northern Isles of Orkney and Shetland. The historical record is incomplete and the kingdom was probably not a continuous entity throughout the entire period. The islands concerned are sometimes referred to as the "Kingdom of Mann and the Isles", although only some of the later rulers claimed that title. At times the rulers were independent of external control, although for much of the period they had overlords in Norway, Ireland, England, Scotland or Orkney. At times there also appear to have been competing claims for all or parts of the territory. The islands involved have a total land area of over 8,300 square kilometres (3,205 sq mi) and extend for more than 500 kilometres (310 mi) from north to south.

Viking influence in the area commenced in the late 8th century, and whilst there is no doubt that the Uí Ímair dynasty played a prominent role in this early period, the records for the dates and details of the rulers are speculative until the mid-10th century. Hostility between the Kings of the Isles and the rulers of Ireland, and intervention by the crown of Norway (either directly or through their vassals the Earls of Orkney) were recurring themes.

Invasion by Magnus Barelegs in the late 11th century resulted in a brief period of direct Norwegian rule over the kingdom, but soon the descendants of Godred Crovan re-asserted a further period of largely independent overlordship. This came to an end with the emergence of Somerled, on whose death in 1164 the kingdom was split in two. Just over a century later the islands became part of the Kingdom of Scotland, following the 1266 Treaty of Perth.

The orthography of the rulers' names is complicated as Old Norse and Gaelic were both spoken throughout the region for much of period under consideration. Thus a single individual might be referred to as Rognvaldr in Icelandic sources, Rag(h)nall in Gaelic, Reginaldus in Latin and perhaps "Rognvald" or "Reginald" in English language sources.

During this period the historical record is particularly sparse and these early entries must be considered as somewhat speculative.

It is also possible that Eiríkr, King of York from 947–948 and 952–5, was a ruler in the islands at some stage in the mid-10th century. Eiríkr is believed by some authorities to be synonymous with the saga character Eric Bloodaxe, although the connection is questioned by Downham (2007), who argues that the former was an Uí Ímair dynast rather than a son of Harald Fairhair. A raid on Northumbria in 949, the purpose of which may have been either to support or oppose the kingship of Amlaíb Cuarán is described as predam albidosorum idem nannindisi in the Chronicle of the Kings of Alba. Alfred P. Smyth translated this as "the raid of the men from beyond the spine of Britain, that is, of the islands."

Various rulers have been identified as ruling Man, but not the Isles as a whole. The Isle of Man may have fallen under Norse rule in the 870s, and paradoxically they may have brought the Gaelic language with them. The island has produced a more densely distributed Viking Age archaeology than anywhere else in the British Isles, but the written records for this time period are poor.

There then follows a period when it is likely that the Western Isles and Mann were jointly held by rulers of the House of Ímar (see above). Downham (2007) suggests Lagmann Godredson may have "wielded power in Man" and possibly even have been king but was expelled sometime after 1005, perhaps by Brian Bóruma. This may indicate that the Earls of Orkney did not control Man itself in the early 11th century. Echmarcach mac Ragnaill and his successors certainly did control Mann, but the extent of their rule over the islands of the Clyde and the Hebrides is not clear. Óláfr mac Lagmann (or Lagmainn) is recorded as having been killed at Clontarf in 1014, fighting with "warriors from the Hebrides".