This thesis examines the saints cults and ecclesiastical landscape of south Wales in the early medieval period, from 600 to 1200, the period following the ‘Age of the Saints’ when the early saints were thought to be active, up to the period when much of the hagiographical material was produced. The first four chapters address the cults of St Cadog, St Illtud, St Teilo and St Dyfrig. Several aspects of the cults are addressed in each chapter. The relationships that the saints and their foundations had with secular rulers and other saints and their cults are examined first. The presence of the cult in the medieval landscape is addressed next, looking at the principal foundations of the saints, the distribution of the dedications to the saints in south Wales and the presence of inscribed stones at these dedications. The relics of the saints are also examined, looking at the main types of relics that were associated with the early medieval saints cults. The cults of St Cadog, St Teilo and St Dyfrig each had a period of promotion in the twelfth century that is also discussed in the respective chapters. The thesis concludes by comparing each of the cults to create an overarching view of how saint’s cults in south Wales developed in this period, and then compares this with the development of the saint’s cults of Ireland, Scotland, Brittany and England, the Insular neighbours of Wales.