The office of Roman Emperor went through a complex evolution over the centuries of its existence. During its earliest phase, the Principate, the reality of autocratic rule was masked behind the forms and conventions of oligarchic self-government inherited from the Roman Republic. The emperor had no specific office unless he chose to occupy the Republican office of consul.
The Julio-Claudian dynasty was composed of the Iulii Caesares and the Claudii Nerones, two distinguished patrician families in the waning days of the old Republic. The Iulii Caesares rose to absolute power in the Roman state in the person of the paterfamilias, Julius Caesar himself; upon his murder in 44 BC, the majority of his estate passed to his posthumously adopted son, Octavian, the grandson of Caesar's sister Julia Minor.
Octavian emerged from a series of civil wars as the sole master of the Roman world, and in January 27 BC was appointed princeps senatus and given the cognomen "Augustus" (Latin, "Majestic" or "Venerable"); henceforth he styled himself "Imperator Caesar Augustus". He continued to be elected consul ordinarius each year until 23 BC.
Historians customarily mark the "First Settlement" of 27 as inaugurating Caesar Augustus's reign as Emperor. This is generally misleading, as his constitutional position that year was little different from his constitutional position as early as July 32 BC (when he provoked war with Cleopatra VII of Egypt as a means of ridding himself of his rival Mark Antony), except that he now held the principate of the senate (an office with chiefly parliamentary and ceremonial functions) and bore an honorific surname.
A far more important development was the "Second Settlement" of 23 BC, when Caesar Augustus accepted tribunicia potestas for life and imperium maius proconsulare. Two further developments concluded the establishment of the Imperial dignity: Caesar Augustus accepted imperium consulare on an ad personam basis in 19 BC and was elected pontifex maximus in 13 BC. Thus the Imperial dignity was fully established as the extraordinary concentration of ordinary powers and immunities.
Caesar Augustus's third wife Livia Drusilla (subsequently "Julia Augusta") had previously borne two children by her first husband, Tiberius Claudius Nero: Tiberius and Drusus. Tiberius's second wife was Julia the Elder, Marcus Agrippa's widow (his first wife had been Vipsania, Agrippa's daughter by his first marriage); Caesar Augustus adopted Tiberius on June 26, 4, whereupon Tiberius himself adopted his brother Drusus's son by Mark Antony's daughter who was also Augustus' niece, Germanicus Julius Caesar. Germanicus married Vipsania Agrippina, Agrippa's daughter by Julia and Tiberius's stepdaughter, and had by her one surviving son, Gaius "Caligula" ("Little Boots"), and a daughter, Julia Agrippina, whose third husband was Germanicus's brother by blood, Claudius (she was his fourth wife); Agrippina had already borne a son, Lucius, whom Claudius adopted under the name Nero in 40; Nero married Claudius's daughter Claudia Octavia in 53.
The year 69 is often called the "Year of the Four Emperors" because it saw four usurpers successively claim the purple robe. The fourth Emperor, coming from Judea, is listed in the next section due to dynastic considerations.
Nero committed suicide on June 9, 68, to escape rebellious soldiers loyal to the disloyal Galba, governor of Hispania Tarraconensis (north-eastern Spain). Galba was deposed in January 69 by a disloyal member of his own entourage, Otho (Nero's governor of Lusitania, i.e., western Spain), who was in turn displaced in April by Vitellius (Nero's governor of Germania Inferior). In late December, Vitellius was deposed by the governor of Judaea, Vespasianus (see below).
The Flavian dynasty was composed of the Flavii Vespasiani, a middle-class family of plebeian stock. A relatively short-lived dynasty of 30 years, the Flavians confirmed the use of "Caesar" to confirm the hereditary nature of the Imperial dignity (Vespasian gave both his sons this rank, and is said to have informed the Senate that one of his sons would succeed him or no one would). Domitian made himself extremely unpopular by his autocratic manner, which was a departure from the traditional fiction that the Emperor was merely first among equals (primus inter pares).
Vespasian's wife Flavia Domitilla bore him a daughter (Flavia Domitilla) and two sons (Titus and Domitian).
The Nervan-Antonine dynasty was a largely artificial one, chiefly built out more of adoption than blood relations, as in the Julio-Claudian or Flavian dynasties (the first Emperor of this dynasty was an elderly, childless man, from the noble Cocceii Nervae). The Nervan-Antonine dynasty produced the famous "Five Good Emperors". The Nervan-Antonine dynasty also marks the first time that an Emperor was depicted with a beard (Emperor Hadrian), and one of the first times that a deceased Emperor was inhumed rather than cremated (Antoninus Pius). Note that the Nervan-Antonine Emperors adopted the regularised style Imperator Caesar NN. Augustus, whereas there had hitherto been considerable variation.
Nerva was a childless bachelor, and as a result adopted the governor of Germania Superior, Trajan, in October 97. Trajan's first cousin once removed, Hadrian, was his ward and governor of Syria at the time of his guardian's death, and acceded to the purple without having been adopted by his predecessor. Hadrian himself adopted Antoninus Pius on February 25, 138; at the same time, Antoninus Pius adopted Marcus Aurelius and Lucius Verus. Marcus Aurelius was son of Trajan's great-grandnephew (and consequently grandson of the half-sister of Hadrian's wife), and subsequently married Antoninus Pius's daughter Annia Galeria Faustina, and Lucius Verus was son of Lucius Ceionius Commodus, who had been Hadrian's first choice as Caesar and Emperor-designate. Marcus Aurelius's sixth son (of eight) was Commodus.
In March 193, the Imperial dignity was quite literally and quite openly auctioned off by the mutinous Praetorian Guard, with Titus Flavius Sulpicianus (father-in-law of the slain Emperor) and Marcus Didius Julianus bidding for the Guard's support for the purple.
Commodus's murder on December 31, 192 was immediately followed the next day by the accession of Pertinax, the praefectus urbi. He was murdered by the Praetorian Guard in late March 193. The consular Didius Julianus was installed by Pertinax's murderers, and was himself murdered on June 1 by a partisan of the rebellious governor of Pannonia Superior, Septimius Severus (see below).
The short-lived Severan dynasty came into the purple primarily not by vote of the Senate like the Julio-Claudii but rather by the point of the sword like the Flavii. The founder of the dynasty, Lucius Septimius Severus, was descended from a provincial family from North Africa and is reputed to have kept his African accent until his death. To help bolster his hold on power, Septimius Severus identified himself with the cause of the late Pertinax. The Antonine Constitution of 212 granted full citizenship to all free men in the Empire.
Septimius Severus's second wife Julia Domna bore him two sons, Lucius "Caracalla" ("Long Coat") and Geta. Caracalla was (falsely) rumoured to have fathered a bastard by his first cousin Julia Soaemias (daughter of his maternal aunt Julia Maesa); this rumoured bastard would later become "Elagabalus" (see below).
Macrinus came from an equestrian family; Dio Cassius writes that he was a Moor from Caesarea. Note that he did not style himself "Caesar", but did add "Severus" to his name and inserted Pius Felix before the title "Augustus". He raised his son Diadumenianus to be co-Emperor with him.
Macrinus was praetorian prefect (praefectus praetorio) under "Caracalla", whom he may have conspired to murder in April 217. His wife Nonia Celsa bore him a son, Diadumenianus, whom he made co-Emperor in 218; both were executed by partisans of "Elagabalus" (see below).
The Severi, in addition to being the second dynasty d'épée, are also the first Roman dynasty to have been restored to the purple. The restoration, however, brought with it a decidedly bizarre character: the first of the restored Severan Emperors, a Syrian historically known as "Elagabalus" (also seen less correctly as "Heliogabalus") was already the hereditary high priest of an Oriental sun god, Elagabal. The restored Severi were also well known for the autocratic power exercised by three Syrian princesses as the éminences grises, or power behind the throne: Elagabalus's mother Julia Soaemias and grandmother Julia Maesa, and Alexander Severus's mother Julia Mamaea.
"Elagabalus" was son of Sextus Varius Marcellus, a Syrian, and Julia Soaemis, daughter of Julia Maesa (the older sister of Julia Domna, wife of Septimius Severus); he was therefore nephew of the late "Caracalla", whose natural son he claimed to be (note that he took the same name as Caracalla upon donning the purple). Elagabalus and Alexander Severus (also seen more correctly as "Severus Alexander") were first cousins; Alexander Severus's mother was Julia Mamaea, another daughter of Julia Maesa.