Because it was legally impossible for a patrician to be a tribune of the plebeians, the first Roman emperor, Caesar Augustus, was offered instead all of the powers of the tribunate without actually holding the office (tribunitia potestas). This formed one of the two main constitutional bases of Augustus' authority (the other was imperium proconsulare maius). It gave him the authority to convene the Senate. Also, he was sacrosanct, had the authority to veto (ius intercessionis), and could exercise capital punishment in the course of the performance of his duties. Most emperors' reigns were dated by their assumption of tribunitia potestas, though some emperors, such as Tiberius, Titus, Trajan, Marcus Aurelius etc, had already received it during their predecessor's reign. Marcus Agrippa and Drusus II, though never emperors, also received tribunitia potestas. By extension from the technical Roman governmental usage, some modern politicians have been called "tribunes of the people."
Since the Plebeian Tribunes and Plebeian Aediles were elected by the Plebeians (commoners that also owned land) in the Plebeian Council, rather than by all of the People of Rome (Plebeians,the aristocratic Patrician class and The "Head Count" (which were not land owning people)), they were technically not "magistrates". While the term "Plebeian Magistrate" (magistratus plebeii) has been used as an approximation, it is technically a contradiction. The Plebeian Aedile functioned as the Tribune's assistant, and often performed similar duties as did the Curule Aediles (discussed above). In time, however, the differences between the Plebeian Aediles and the Curule Aediles disappeared.
Since the Tribunes were considered to be the embodiment of the Plebeians, they were sacrosanct. Their sacrosanctity was enforced by a pledge, taken by the Plebeians, to kill any person who harmed or interfered with a Tribune during his term of office. All of the powers of the Tribune derived from their sacrosanctity. One obvious consequence of this sacrosanctity was the fact that it was considered a capital offense to harm a Tribune, to disregard his veto, or to interfere with a Tribune. The sacrosanctity of a Tribune (and thus all of his legal powers) were only in effect so long as that Tribune was within the city of Rome. If the Tribune was abroad, the Plebeians in Rome could not enforce their oath to kill any individual who harmed or interfered with the Tribune. Since Tribunes were technically not magistrates, they had no magisterial powers ("major powers" or maior potestas), and thus could not rely on such powers to veto. Instead, they relied on the sacrosanctity of their person to obstruct. If a magistrate, an assembly or the senate did not comply with the orders of a Tribune, the Tribune could 'interpose the sacrosanctity of his person' (intercessio) to physically stop that particular action. Any resistance against the Tribune was tantamount to a violation of his sacrosanctity, and thus was considered a capital offense. Their lack of magisterial powers made them independent of all other magistrates, which also meant that no magistrate could veto a Tribune.
Tribunes could use their sacrosanctity to order the use of capital punishment against any person who interfered with their duties. Tribunes could also use their sacrosanctity as protection when physically manhandling an individual, such as when arresting someone. On a couple of rare occasions (such as during the tribunate of Tiberius Gracchus), a Tribune might use a form of blanket obstruction, which could involve a broad veto over all governmental functions. While a Tribune could veto any act of the senate, the assemblies, or the magistrates, he could only veto the act, and not the actual measure. Therefore, he had to physically be present when the act was occurring. As soon as that Tribune was no longer present, the act could be completed as if there had never been a veto.
Tribunes, the only true representatives of the people, had the authority to enforce the right of Provocatio, which was a theoretical guarantee of due process, and a precursor to the common law concept of habeas corpus. If a magistrate was threatening to take action against a citizen, that citizen could yell "provoco ad populum", which would appeal the magistrate's decision to a Tribune. A Tribune had to assess the situation, and give the magistrate his approval before the magistrate could carry out the action. Sometimes the Tribune brought the case before the College of Tribunes or the Plebeian Council for a trial. Any action taken in spite of a valid provocatio was on its face illegal. In this capacity, the Tribune was the principal, and often the only, guanentor of the civil liberties of Roman citizens against arbitrary state power. The degree of liberty afforded to Roman citizens by the Tribune through the power of Provocatio was unmatched in the ancient world.
Tribunes were required to be plebeians, and until 421 BC this was the only office open to them. In the late Republic the patrician politician Clodius arranged for his adoption by a plebeian branch of his family, and successfully ran for the tribunate. When Lucius Cornelius Sulla was dictator he severely curtailed the tribunes of the plebeians by invalidating their power of veto and making it illegal for them to bring laws before the Concilium Plebis without the Senate's consent. Afterwards, the tribune was restored to its former power during the consulship of Crassus and Pompey.
Throughout the Republic and its fall, powerful individuals used the tribunes for their personal glory and gain. Clodius and Milo were both tribunes who used violence in the courts and government in order to achieve the needs and requests of Pompey and Caesar. When the Senate refused to grant Caesar's veterans lands and a further governorship of Gaul, he turned to the tribunes with his demands and got them.
All middle-ranking officers of the legions were also titled tribunes, though they were unelected and junior to the tribunum militi. Messala, the villain in the 1880 novel Ben-Hur by Lew Wallace and its 1959 film, was a military tribune.
It was instituted by Napoleon I Bonaparte's constitution of the revolutionary year VIII "in order to moderate the other powers" by discussing every legislative project and sending its orateurs ("orators", i.e. spokesmen) to defend or attack them in the Corps législatif. Its 100 members were designated by the Senate from the list of citizens from 25 years up, and annually one fifth was renewed for a five-year term.
When it opposed the first parts of Bonaparte's proposed penal code, he made the Senate nominate 20 new members at once to replace the 20 first opponents to his politic; they accepted the historically important reform of penal law. As the Tribunate opposed new despotic projects, he got the Senate in year X to allow itself to dissolve the Tribunat. In XIII it was further downsized to 50 members. On August 16, 1807 it was abolished and never revived.