Medieval Wales had its own system of law, separate from the Common Law in England, and from Canon Law. The law is attributed to King Hywel ap Cadell, or Hywel Dda (Hywel the Good), d. 949 or 950. There is very little concrete evidence to connect the tenth-century ruler with the existing manuscripts of Welsh law, but Hywel may have developed some aspects of legal activity in Wales in order to gain the (possibly later) attribution as lawmaker. The story of his legal activity in the prologues to the Welsh law manuscripts was probably created to give the law a royal and ecclesiastical basis, a defence against the criticisms of Norman clerics.

Cyfraith Hywel (‘the law of Hywel’) survives in over 40 manuscripts, which separate into groups with common attributes. The lengthy manuscripts were intended to present the law in its entirety – this was a codified legal system. The law texts all follow the same basic structure, arranged into subject-specific sections or ‘tractates’. Dating the law is problematic due to the disparity between the text and the manuscripts. The tractates, and even material within them, can be a mixture of early and late additions, as the law was an ever-evolving entity. However, it can be said that Cyfraith Hywel reached its pinnacle in Gwynedd, north-west Wales, during the thirteenth century, particularly during the reigns of Llywelyn ab Iorwerth and his grandson, Llywelyn ap Gruffudd. The earliest copies of the laws date from this period, and it is no accident that the golden age for the law and the existence of these earliest copies coincide chronologically.

With the conquest of Wales by Edward I in 1282, aspects of the law were abolished – according to the Statute of Wales in 1284, English law was to be followed in criminal cases, but Cyfraith Hywel could remain in use in other matters. But this was not to be the end of Cyfraith Hywel, as it continued in use in particular regions, including in south Wales in territories that were not conquered by Edward I, where Welsh law remained in use albeit in the context of a different legal process.