The history of illegitimacy has been much studied in England, Scotland and elsewhere in Europe, but has attracted little attention in Wales, in spite of the significance of the debate about the sexual laxity of Welsh courting couples to the historiography of the nineteenth century. This thesis examines illegitimacy in the counties of Glamorgan and Carmarthenshire between 1660 and 1870, using data from 74 parishes to measure the changing level of illegitimacy, at a time when south Wales was being transformed by economic, social and religious change. The research sets out to introduce Wales to the debate on illegitimacy, locating south Wales within the established picture of European demography. Patterns of illegitimacy in England and Wales are compared in order to establish whether south Wales fell within the range of regional variation found in England or displayed a distinct pattern, related to different traditions of courtship and marriage, or to experiences such as industrialisation and religious revival, which developed in different ways in the two countries. Material from poor law records, the consistory courts, Nonconformist disciplinary records, and personal narratives is brought together to give a holistic picture of the courtship of young people, and the circumstances of illegitimate children and their parents. Analysis suggests that patterns of illegitimacy in south Wales fit well with the characteristics of the ‘highland’ region of England described by Adair, but with additional intra-regional variation in levels of illegitimacy, which reflected local social, economic and cultural factors.