Vetranio (d. c. 360), born in the province of Moesia in a part of the region located in modern Serbia, is sometimes but incorrectly referred to as Vetriano. He was an experienced soldier and officer when he was asked by Constantina, the sister of Roman Emperor Constantius II, to proclaim himself Caesar (March 1, 350). Her brother Constans had been killed by Magnentius earlier that year and she probably thought Vetranio could protect her family and herself against the usurper. Vetranio accepted and coins were minted in his name, showing the title of Augustus (full emperor), rather than Caesar. Constantius first seemed to accept the new Emperor and sent him money to raise an army, as well as his regalia.

Constantius, who was on a campaign against the Persians when Magnentius came to power, returned to the West and met with Vetranio. Vetranio subsequently abdicated on December 25, 350. He was allowed to live the remainder of his years as a private citizen on a state pension in Prusa ad Olympium, Bithynia.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.

Flavius Valentinianus (371 – 15 May 392), commonly known as Valentinian II, was Roman Emperor from 375 to 392 A.D.

Flavius Valentinianus was born to Emperor Valentinian I and his second wife, Justina. He was the half-brother of Valentinian’s other son, Gratian, who had shared the imperial title with his father since 367. He had three sisters Galla, Grata and Justa. The elder Valentinian died on campaign in Pannonia in 375. Neither Gratian (then in Trier) nor his uncle Valens (emperor for the East) were consulted by the army commanders on the scene. Instead of merely acknowledging Gratian as his father’s successor, Valentinian I’s generals acclaimed the four-year old Valentinian augustus on 22 November 375. The army, and its Frankish general Merobaudes, may have been uneasy about Gratian's lack of military ability, and so raised a boy who would not immediately aspire to military command.

Gratian, forced to accommodate the generals who supported his half-brother, governed the trans-alpine provinces (including Gaul, Hispania, and Britain), while Italy, part of Illyricum, and Africa were under the rule of Valentinian. In 378, their uncle, the Emperor Valens, was killed in battle with the Goths at Adrianople, and Gratian invited the general Theodosius to be emperor in the East. As a child, Valentinian II was under the influence of his Arian mother, the Empress Justina, and the imperial court at Milan, an influence contested by the Catholic bishop of Milan, Ambrose.

Valentinian I (Latin: Flavius Valentinianus Augustus; 321 – 17 November 375), also known as Valentinian the Great, was Roman emperor from 364 to 375. Upon becoming emperor he made his brother Valens his co-emperor, giving him rule of the eastern provinces while Valentinian retained the west.

During his reign, Valentinian fought successfully against the Alamanni, Quadi, and Sarmatians. Most notable was his victory over the Alamanni in 367 at the Battle of Solicinium. His brilliant general Count Theodosius defeated a revolt in Africa and the Great Conspiracy, a coordinated assault on Britain by Picts, Scots, and Saxons. Valentinian was also the last emperor to conduct campaigns across both the Rhine and Danube rivers. Valentinian rebuilt and improved the fortifications along the frontiers – even building fortresses in enemy territory.

Due to the successful nature of his reign and almost immediate decline of the empire after his death, he is often considered the "last great western emperor". He founded the Valentinian Dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.