thumb|right|Map of the extent of Hywel Dda's power
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 grandson 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.
Hywel is highly esteemed among other medieval Welsh rulers. His name is particularly linked with the codification of traditional Welsh law, which were thenceforth known as the Laws of Hywel Dda. The latter part of his name (Dda, <small>lit.</small> "Good") refers to the fact that his laws were just and good. The historian Dafydd Jenkins sees in them compassion rather than punishment, plenty of common sense and recognition of the rights of women. Hywel Dda was a well-educated man even by modern standards, having a good knowledge of Welsh, Latin and English.
The office building and original home of the Senedd is named Tŷ Hywel ("Hywel House" or "Hywel's House") in honour of Hywel Dda. The original assembly chamber, now known as Siambr Hywel ("Hywel's Chamber"), is used for educational courses and for children and young people's debates. The local health board of south-west Wales, covering an area roughly corresponding to the kingdoms of Dyfed and Seisyllwg of which Hywel was King, also bears his name.
Early life
Hywel was the son of King Cadell ap Rhodri of Seisyllwg. He had a brother, Clydog ap Cadell, who was probably the younger of the two. Hywel was later reputed to have married Elen ferch Llywarch (893-943), the supposed heiress of King Llywarch ap Hyfaidd of Dyfed,
Hywel's name is associated with the laws of medieval Wales, which are commonly known as the Laws of Hywel Dda (Welsh: Cyfraith Hywel). None of the law manuscripts can be dated to Hywel's time, but Hywel's name is mentioned in the prologues to the laws, and are also known as the Code of Dyfed. These describe how Hywel gathered expert lawyers and priests from each commote in Wales together in the White Land in Dyfed () in order to revise and codify the Laws of Wales.