Hywel ap Cadell, commonly known as Hywel Dda, which translates to Howel the Good in English, was a Welsh king who ruled the southern Welsh kingdom of Deheubarth and eventually came to rule most of Wales. He became the sole king of Seisyllwg in 920 and shortly thereafter established Deheubarth, and proceeded to gain control over the entire country from Prestatyn to Pembroke. As a descendant of Rhodri Mawr through his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty. He was recorded as King of the Britons in the Annales Cambriæ and the Annals of Ulster.

Deheubarth was a regional name for the realms of south Wales, particularly as opposed to Gwynedd. It is now used as a shorthand for the various realms united under the House of Dinefwr, but that Deheubarth itself was not considered a proper kingdom on the model of Gwynedd, Powys, or Dyfed is shown by its rendering in Latin as dextralis pars or as Britonnes dexterales and not as a named land. In the oldest British writers, Deheubarth was used for all of modern Wales to distinguish it from Hen Ogledd, the northern lands whence Cunedda originated.