The Roman Senate (Latin senātus, from senex, old) grew out of a body which advised the kings who ruled Rome for the first two-and-a-half centuries of its history (753-509 BCE). After the expulsion of the last king, Tarquinius Superbus (reigned 535-509 BCE) and the establishment of the Republic, the Senate gradually became the most authoritative institution in the state. (The letters SPQR which appear on the standards (vexilla) of the Roman legions to indicate their allegiance to the state of Rome are an abbreviation of Senatus populusque Romanus, ‘the Senate and people of Rome’.) Formally the Senate was still no more than an advisory body, its function being to advise the magistrates (i.e., the holders of political office), assign them specific duties, and discuss their legislative proposals: the election of magistrates and the right to enact legislation lay with the comitia or popular assembly. The source of the Senate’s authority, enabling it to exercise a decisive influence on all aspects of domestic and foreign policy, was the status of its members, almost all of them ex-magistrates, and many of them with considerable experience and expertise in affairs of state. Members of the Senate also sat as judges in both the civil and the criminal courts. Senators were distinguished by their dress: they wore distinctive shoes made of red leather and had the latus clavus (a broad purple stripe running perpendicularly from the neck) woven into the fabric of their tunics. During the final years of the Republic the authority of the Senate was challenged and undermined by powerful military leaders (such as Julius Caesar) who, with large armies at their command, were able to bend the Senate to their will.
Under the Roman Empire (27 BCE-476 CE) the Senate remained, but its authority was much diminished. While it acquired the right to elect the magistrates and had responsibility for the government of Italy and for those provinces of the Empire which required only small military garrisons, the emperor soon gained control of the treasury (aerarium) and could always intervene in elections, either by recommendation or nomination, to secure the appointment of his favoured candidates. In the later years of the Empire membership of the Senate became virtually hereditary, and the senators lacked the spirit of public service which had motivated (many of) their Republican predecessors: their only concern was to preserve the Empire and their own privileged status within it.