Yan Thomas taught at the École des Hautes Études (Paris) until his death in 2008 at the age of 56.Truly seminal as a specialist in Roman and legal history, Thomas left a work that consists mainly of numerous articles.His work is best described as an inventory of genealogical prolegomena to the understanding of the Western legal tradition.The essays collected here provide a clear insight into the stakes of Thomas' renowned inquiries.Thomas’ investigations into Roman history and law focus, case-by-case, upon the law’s operations – and, by extension, upon their expansion and transmission.