The gens Aemilia, originally written Aimilia, was one of the most ancient patrician houses at Rome. The family was said to have originated in the reign of Numa Pompilius, the second King of Rome, and its members held the highest offices of the state, from the early decades of the Republic to imperial times. The Aemilii were probably one of the gentes maiores, the most important of the patrician families. Their name was associated with two major roads (the Via Aemilia and the Via Aemilia Scauri), an administrative region of Italy, and the Basilica Aemilia at Rome.
Several stories were told of the foundation of the Aemilii. The most familiar was that their ancestor, Mamercus, was the son of Numa Pompilius, who was also claimed as an ancestor of the gentes Pompilia, Pomponia, Calpurnia, and Pinaria. A variation of this account stated that Mamercus was the son of Pythagoras, who was sometimes said to have taught Numa. However, as Livy observed, this was not possible, as Pythagoras was not born until more than a century after Numa's death, and was still living in the early days of the Republic.
This Mamercus is said to have received the name of Aemilius because of the persuasiveness of his language (δι αιμυλιαν λογου), although such a derivation is certainly false etymology. Another possible derivation is from aemulus, "a rival," or from the same root. According to a different legend, the Aemilii were descended from Aemylos, a son of Ascanius, four hundred years before the time of Numa Pompilius. Still another version relates that the gens was descended from Amulius, the wicked uncle of Romulus and Remus, who deposed his brother Numitor to become king of Alba Longa.