This article sheds new light on illegitimacy in eighteenth-century Britain through an analysis of evidence from 36 parishes across the former Welsh counties Montgomeryshire and Radnorshire. Quantitative analysis of illegitimacy ratios demonstrates that levels were significantly higher in certain, but not all, parts of Wales in the eighteenth century. This evidence is considered in relation to explanatory frameworks used in the analysis of English data, which attempt to account for rising levels through cultural changes that influenced premarital sexual behaviour, and economic opportunities created by industrialization. Welsh evidence appears to present a challenge to these understandings in two key ways: Wales was linguistically different and lacked certain cultural markers which some historians have associated with an eighteenth-century 'sexual revolution', and because the highest levels of illegitimacy were found in agricultural regions of Wales which experienced little or no industrial change. It is argued that Welsh illegitimacy was influenced by a combination of courtship-led marriage customs, a decline in traditional forms of social control and worsening economic circumstances which, on closer examination, appear remarkably similar to London. This analysis provides further evidence that illegitimacy in eighteenth-century Britain was a deeply complex phenomenon governed by diverse regionally specific social and economic influences.