Tribunes were required to be plebeians, and until 421 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.
By extension from the technical Roman governmental usage, some modern politicians have been identified as "Tribunes of the People." .
Throughout the Republic and its fall, certain 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. Further more, when the Senate refused to grant Caesar all his requests he turned to the tribunes to grant him all he wanted -- ie. Pompey's veterans lands and him a further governship of Gaul. Again violence was used against those tribunes that prevented their quest for glory.
Because it was legally impossible for a patrician to be a tribune of the people, the first Roman "emperor", Caesar Augustus, was offered instead all of the powers of the tribunate without actually holding the office (tribunicia potestas). This formed the constitutional basis of the emperor's authority; he was sacrosanct, had ius intercessio, and could exercise capital punishment in the course of the performance of his duties. As a result, there was never actually an "office" of emperor; emperors' reigns were dated by their assumption of tribunicia potestas, and the actual constitutional position of the emperor was that of pontifex maximus (P.M.) tribunicia potestate (trib. pot.).