List of Roman emperors and principal dynasties

Succession, legitimacy and common causes of change

Sovereignty in the Roman world combined dynastic claims, adoption, senatorial acknowledgment and above all support from the army. Emperors could be chosen by troops, named by predecessors, or emerge after civil conflict; usurpation, assassination and military acclamation were recurring causes of change. The result is a chronology marked by long dynasties, short-lived reigns and episodic crises—periods when multiple claimants competed for control and provincial power brokers influenced succession.

Principal dynasties and representative emperors

• Julio–Claudian dynasty – The principate began with Augustus, who ended the civil wars and established core institutions. He was followed by Tiberius, then the turbulent reigns of Caligula, Claudius (under whom Britain was annexed) and Nero, whose fall led to a brief interlude of civil struggle.
• Rapid succession and civil war – In the decades after Nero the empire experienced episodes of fast turnover, such as the Year of the Four Emperors, in which military commanders and provincial legions backed competing candidates. Figures like Galba and Vitellius appeared briefly before the army selected a stabilizing ruler.
• Flavian dynasty – After civil war, the army elevated Vespasian, whose family completed large public works including the Colosseum. His sons Titus and Domitian ruled in succession; Domitian continued campaigns on the northern frontiers, including activity in southern Scotland.
• Nerva–Antonine era – A comparatively stable period often associated with the adoption system of succession. The era and its institutions are sometimes grouped as the Nerva–Antonine dynasty. Emperors included Nerva, the expansionist Trajan (under whom the empire reached its greatest extent), and Hadrian, builder of Hadrian's Wall to guard against the northern Picts in Roman Britain. Other notable rulers of this period were Antoninus Pius, Lucius Verus, Marcus Aurelius and Commodus.
• Early third century and regional competition – Political pressure and external threats produced frequent turnovers and military emperors. Events such as the Year of the Five Emperors and the rise of the Severan dynasty exemplify the changing balance between Rome, the army and provincial elites.
• Crisis of the Third Century – The mid-3rd century saw fragmentation, economic stress and many short reigns during the Crisis of the Third Century. Emperors and claimants included Gordian I, Aemilianus, Aurelian and Tacitus, among numerous others. Provincial commanders frequently became emperors by military acclamation, and breakaway regimes briefly controlled parts of the empire.

Responses to the crisis included administrative and fiscal reforms, and ultimately a reconfiguration of imperial rule in the late third century. Diocletian established the tetrarchy, dividing authority among co-emperors to improve defense and governance; his colleagues and successors included Galerius. The tetrarchic structure evolved, and one of its later beneficiaries was Constantine, who defeated rivals such as Licinius, became sole ruler and promoted Christianity while reorganizing the imperial administration.

Late empire, division and continuation

After Constantine the empire continued to be ruled by successive dynasties and military potentates. The Valentinian and Theodosian houses sought to defend the frontiers and manage relations with migrating peoples. Theodosius I is notable for the administrative and religious settlement that effectively left the empire divided between eastern and western administrations; over time the western polity weakened under pressure from various Germanic groups. The traditional end of the Western Roman Empire is placed in the late fifth century, while the eastern court endured as the Byzantine or Eastern Roman polity; historians therefore distinguish the Western Roman Empire from its eastern continuation to clarify political and cultural divergence.

Comprehensive chronological lists of emperors enumerate mainline rulers, junior colleagues, and the many usurpers and fleeting claimants whose actions shaped imperial history. Useful study approaches include chronological tables, dynastic lists and prosopographical works that note dates, co-emperors and the circumstances of succession. For those researching individual reigns, primary sources, numismatic evidence and modern syntheses together provide the best basis for assessing the life, power base and policies of each emperor.

Note that besides the regular succession of emperors, a large number of local usurpers and short-lived rival claimants appeared at different times; some controlled whole provinces for years, others vanish from the record after weeks. The complexity of imperial chronology therefore requires careful use of sources and attention to regional developments when compiling full lists.