The paper examines the concept of fidelitas (fealty) in the ‘Libri Feudorum', a twelfth- to thirteenth-century northern Italian compilation of legal texts on fiefs and vassals. There, the expression is used in two ways: in a narrower sense as a term for the vassalic oath, and in a wider one to describe the relationship between a lord and his man. It will be argued that by collecting the various references and different meanings of fidelitas, a better understanding of thirteenth-century feudo-vassalic relations as reflected by the ‘Libri Feudorum' can be reached. Their striking feature is the legally normative connotation of fidelitas: according to the ‘Libri Feudorum', the relationship between lord and vassal is based on an agreement under oath, whereby two parties swear to comply with the norms explicitly mentioned in the agreement and furthermore to follow regional customary law regarding vassalage, all undertaken by the vassal in order to obtain a fief. The fidelitas of the ‘Libri Feudorum' is thus set apart from more extensive notions of general loyalty towards a person (Gesinnungstreue) and limited in its meaning to contractual fidelity (Vertragstreue) under feudal law. This observation underscores the increasing importance legal normativity gained in the course of the high and late Middle Ages as a means to define feudo-vassalic relations.