This article aims to describe the reasons for the decline of customary law in the early modern era. Confining the discussion to a limited geographical setting – the Iberian Peninsula – the arguments I used might be easily applied to other European jurisdictions. Part I presents an explanation of the predominance of custom in the medieval Spanish legal traditions. Part II describes the general features of the law in the early modern era, since they contributed – to a greater or lesser degree – to the demise of custom. Part III focuses more specifically on the theoretical and practical reasons for the decline and displacement of custom in early modern Spain. Part IV describes the consequences of the Decrees of Nueva Planta (1707-1718), approved by Felipe V in the context of the War of the Spanish Succession (1700-1714), regarding the development of the notion and role of custom in the eighteenth century. The article concludes with some reflections, emphasising that although customs do not easily co-exist with the state or a strong political power, neither do they entirely perish.