The executive magistrates of the Roman Empire were elected individuals of the ancient Roman Empire. During the transition from monarchy to republic, the constitutional balance of power shifted from the executive (the Roman King) to the Roman Senate. During the transition from republic to empire, the constitutional balance of power shifted back to the executive (the Roman Emperor). Theoretically, the senate elected each new emperor, although in practice, it was the army which made the choice. The powers of an emperor, (his imperium) existed, in theory at least, by virtue of his legal standing. The two most significant components to an emperor's imperium were the "tribunician powers" (potestas tribunicia) and the "proconsular powers" (imperium proconsulare). In theory at least, the tribunician powers (which were similar to those of the Plebeian Tribunes under the old republic) gave the emperor authority over Rome's civil government, while the proconsular powers (similar to those of military governors, or Proconsuls, under the old republic) gave him authority over the Roman army. While these distinctions were clearly defined during the early empire, eventually they were lost, and the emperor's powers became less constitutional and more monarchical.
Under the empire, the citizens were divided into three classes, and for members of each class, a distinct career path was available (known as the cursus honorum). The traditional magistracies were only available to citizens of the senatorial class. The magistracies that survived the fall of the republic were (by their order of rank in the cursus honorum) the Consulship, Praetorship, Plebeian Tribunate, Aedileship, Quaestorship, and Military Tribunate. If an individual was not of the senatorial class, he could run for one of these offices if he was allowed to run by the emperor, or otherwise, he could be appointed to one of these offices by the emperor. Mark Antony abolished the offices of Roman Dictator and Magister equitum ("Master of the Horse") during his Consulship in 44 BC, shortly after the assassination of Julius Caesar. The office of Interrex was also abolished during the transition from republic to empire. In 22 BC the emperor Augustus appointed P. Aemilius Lepidus and L. Munatius Plancus to the Censorship, and while they began a census that year, they never completed it, and shortly thereafter the office was abolished. The emperor Claudius attempted to revive the office by appointing himself and L. Vitellius Censor in 47 AD, but after Claudius, no further attempts were made to revive the office.
Roman Emperor
By virtue of his proconsular powers, the emperor held the same grade of military command authority as did the chief magistrates (the Roman Consuls and Proconsuls) under the republic. Since republican Proconsuls had often held their authority for extended periods of time, the prolonged use of this power by the emperor did have precedent. However, the emperor was not subject to the constitutional restrictions that the old Consuls and Proconsuls had been subject to. For example, he was not required to observe collegiality, since he had no colleague, and he could not have his actions vetoed. Eventually, he was given powers that, under the republic, had been reserved for the Roman Senate and the Roman assemblies, including the right to declare war, to ratify treaties, and to negotiate with foreign leaders. The emperor's degree of Proconsular power gave him authority over all of Rome's military governors. Under the republic, Proconsuls (usually former Consuls) were made governors of the more challenging provinces, and as such, most of the Roman army was under the command of one of the Proconsuls. In contrast, the "Propraetors" (usually former Praetors) were made governors of the more stable provinces. Under the republic, Praetors were the second highest ranking magistrates after the Consuls, which was why Propraetors were given provinces that were more stable. Under the early empire, the emperor commanded these Proconsular provinces, while the senate commanded the more stable Propraetorial provinces. It was by this that the emperor held command authority over most of the Roman army.