The gens Cassia was a Roman family of great antiquity. The gens was originally patrician, but all of the members who appear in later times were plebeians. The first of the Cassii to obtain the consulship was Spurius Cassius Viscellinus, in 502 BC. He was the proposer of the first agrarian law, and was put to death by the patricians. As all of the Cassii known from after his time are plebeians, it is not improbable either that the patricians expelled them from their order, or that they abandoned it on account of the murder of Viscellinus.
The Cassia gens was reckoned one of the noblest in Rome; and members of it are constantly mentioned under the Empire as well as during the Republic. The Roman road to Arretium was called the Via Cassia, and the village of Cassianum Hirpinum was named for an estate of the family in the country of the Hirpini. One family of the Cassii was one of the dominant houses of Olissipo in Lusitania.
A possible clue to the origin of the Cassii is the cognomen Viscellinus or Vecellinus, borne by the eldest branch of the family. It appears to be derived from the town of Viscellium or Vescellium, a settlement of the Hirpini, which is mentioned by Titus Livius in connection with the Second Punic War. The town was one of three captured by the praetor Marcus Valerius Laevinus after they had revolted in 215 BC. Its inhabitants, the Viscellani, are also mentioned by Plinius. This suggests the possibility that the ancestors of the Cassii were from Hirpinum, or had some other connection with Viscellium. The existence of a substantial estate of the Cassii in Hirpinum at a later time further supports such a connection.