Roman imperial dynasties
The Tetrarchy
Portrait of the Four Tetrarchs, two porphyry sculptures looted from the Philadelphion of Constantinople after 1204, now standing at the southwest corner of St Mark's Basilica, Venice
Diocletian as Augustus 284–286
— with Maximian as Caesar 285–286
Maximian & Diocletian
as Augusti of the West and East
— with Constantius I & Galerius
as Caesares
Constantius I & Galerius
as Augusti of West and East
— with Severus II & Maximinus II
as Caesares
Severus II and Galerius as Augusti of West and East 306–307
— with Maximinus II and Constantine I as Caesares
Maxentius & Maximian as usurpers in Italy and Africa 306–308
Galerius as Augustus 307–308
— with Maximinus II as Caesar
— with Constantine I as
self-proclaimed Augustus
Licinius & Galerius
as Augusti of West and East
— with Maximinus II and Constantine I as Caesares
Maxentius as usurper in Rome (and Asia Minor 311–312) 308–312
Licinius I & Maximinus II
as Augusti of West and East
— with Constantine I as
self-proclaimed Augustus
Constantine I & Licinius I
as Augusti of West and East
— with Licinius II, Constantine II & Crispus as Caesares
— with Valerius Valens as
Augustus of the West
— with Martinian as
Augustus of the West
Preceded by
Crisis of the Third Century
Followed by
Constantinian dynasty

The Tetrarchy was the system instituted by Roman emperor Diocletian in 293 AD to govern the ancient Roman Empire by dividing it between two emperors, the augusti, and their junior colleagues and designated successors, the caesares.

Initially Diocletian chose Maximian as his caesar in 285, raising him to co-augustus the following year; Maximian was to govern the western provinces and Diocletian would administer the eastern ones. The role of the augustus was likened to Jupiter, while his caesar was akin to Jupiter's son Hercules. Galerius and Constantius were appointed caesares in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the rank of augustus. Their places as caesares were in turn taken by Valerius Severus and Maximinus Daza.

The orderly system of two senior and two junior rulers endured until Constantius died in July 306, and his son Constantine was unilaterally acclaimed augustus and caesar by his father's army. Maximian's son Maxentius contested Severus' title, styled himself princeps invictus, and was appointed caesar by his retired father in 306. Severus surrendered to Maximian and Maxentius in 307. Maxentius and Constantine were both recognized as augusti by Maximian that same year. Galerius appointed Licinius augustus for the west in 308 and elevated Maximinus Daza to augustus in 310.

Constantine's victory over Maxentius at the Battle of the Milvian Bridge in 312 left him in control of the western part of the empire, while Licinius was left in control of the east on the death of Maximinus Daza. Constantine and Licinius jointly recognized their sons – Crispus, Constantine II, and Licinius II – as caesares in March 317. Ultimately the tetrarchic system lasted until c. 324, when mutually destructive civil wars eliminated most of the claimants to power: Licinius resigned as augustus after losing the Battle of Chrysopolis, leaving Constantine in control of the entire empire.

The Constantinian dynasty's emperors retained some aspects of collegiate rule; Constantine appointed his son Constantius II as another caesar in 324, followed by Constans in 333 and his nephew Dalmatius in 335, and the three surviving sons of Constantine in 337 were declared joint augusti together, and the concept of the division of the empire under multiple joint emperors endured until the Fall of the Western Roman Empire. In the Eastern Roman Empire, augusti and caesares continued to be appointed sporadically.