The emperors were able to add individuals to the senatorial rolls due to their censorial powers. The censorship had been formally abolished by Augustus in 22, after several periods during which the censorship either lapsed or was written out entirely. With that Augustus adopted censorial powers, as Caesar had before him.

The senate existed as an advisory body with few actual legal powers. Traditionally it had first been established by the kings as a group of elder advisors. In the very earliest Republic traditionally the consuls had simply appointed senators at will, but a lex Ovinia in 312 transferred the right to the censors, which had been established in 443 by a lex de creandis censoribus. Since the senate was an advisory body without particularly clear powers the requirements for admission were not especially clear either. Generally speaking an admitted senator had to have rendered noteworthy service to the people and state, had to be of proper social standing, and had to meet the minimum property qualifications, which by the late Republic was essentially a prerequisite for magisterial election. Change in any of these could be cause to remove an individual from the senatorial rolls. The exact process was relatively straightforward: as part of the census every five years the censors were obliged (provided a census actually took place, which was far from certain by the first century) to put together a new roll of senators, and subjected them to some questioning before entering their names into the list. For most of the middle Republic election to a curule magistracy led essentially to automatic enrollment in the senate once the fasces were put down. Sulla made it standard practice to enroll the quaestors into the senate as well, the plebeian aediles having joined their curule colleagues sometime before the end of the second century. This was later formally made law sometime later. The senate could also technically co-opt individuals into their ranks, like the priestly colleges, though this was rarely done in the Republic. Beginning especially under Tiberius, when free elections disappeared, cooption became the normal means of entry into the senate, as the magistrates were being chosen by the senate from their own number and therefore there were no more quaestors being elected and standing in the senate after their year of office.