Yan Thomas taught at the cole des Hautes tudes (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.