The present work is divided into three main parts and it is a study of the concept of auctoritas. In the first part, I described the period of Rome’s foundation, when the idea of auctoritas appeared in the history of Rome, right after Romulus had received the gods’ authorization to found a new city. I enquired into etymology and I examined the translation of auctoritas in other languages, in the Greek language most of all. Using a philological method, I wrote a commentary on the use of the word in the classical authors, from Plautus to Cicero. In these literary sources, the use of auctoritas appears coherent in the hendiadys with Senatus and patrum or patres. In the second part, the thesis investigated the legal sources, by XII Tables to Digestum, to prove the conceptual order that remained unchanged along the passage of time. Auctoritas, in these sources, is related to authorization of a legal act and it increases the legal entity of the person appealing to it. In the imperial age, Auctoritas meant the authorization strength of the law. In every historical age, Auctoritas keeps its original aspect from Rome’s foundation and its religious context.