Guillaume Durand (c. 1230–1296) is known as the Speculator, from the title of the work – Speculum iudiciale – which earned him the reputation as one of the most authoritative jurists of the European Middle Ages. He is certainly to be considered one of the fathers of the science of European procedural law, but his intellectual figure was multifaceted, ranging from theology to liturgy and law. The chapter is mainly concerned with the Speculum iudiciale and aims to show that this work is not merely a treatise on procedural law, because the “mirror of procedure” is able to return the image of the deep roots of medieval Christian conceptions of law. In particular, the chapter explores the theme of due process and, in showing the relation of Durand’s thought with the medieval tradition of canon law and theology, highlights the influence that the ideas emerging in the medieval debate have had in the formation of the juridical culture of the Western world.