Didius Julianus (Latin: Marcus Didius Severus Julianus Augustus; 30 January 133 or 2 February 137 – 1 June 193), was Roman Emperor for three months during the year 193. He ascended the throne after buying it from the Praetorian Guard, who had assassinated his predecessor Pertinax. This led to the Roman Civil War of 193–197. Julianus was ousted and sentenced to death by his successor, Septimius Severus.

Julianus was born to Quintus Petronius Didius Severus and Aemilia Clara. Julianus's father came from a prominent family in Mediolanum (Milan) and his mother was an African woman, of Roman descent. Clara came from a family of consular rank. His brothers were Didius Proculus and Didius Nummius Albinus. His date of birth is given as 30 January 133 by Cassius Dio and 2 February 137 by the Historia Augusta. Didius Julianus was raised by Domitia Lucilla, mother of Roman Emperor Marcus Aurelius. With Domitia's help, he was appointed at a very early age to the vigintivirate, the first step towards public distinction. He married a Roman woman called Manlia Scantilla and about 153, Scantilla bore him a daughter and only child Didia Clara.

Flavius Petronius Maximus (possibly Flavius Anicius Petronius Maximus) (c. 396 – 31 May 455) was Western Roman Emperor for two and one-half months in 455. A wealthy senator and a prominent aristocrat, he was instrumental in the murders of the Western Roman magister militum, Flavius Aëtius, and the Western Roman Emperor Valentinian III. Maximus was killed during the events that culminated in the Vandal sack of Rome in 455.

Petronius Maximus was born in about 396. Although he was of obscure origin, it is now believed that he belonged to the Anicii family. Related to later Emperor Olybrius, Maximus was the son of Anicius Probinus, the son of Anicia Faltonia Proba and Sextus Claudius Petronius Probus, who was Prefect of Illyricum in 364, Prefect of Gaul in 366, Prefect of Italy in 368–375 and again in 383 and consul in 371.

Maximus achieved a remarkable career early on in his life. His earliest known office was praetor, held about 411; around 415 he served as a tribunus et notarius, which was an entry position to the imperial bureaucracy, and led to his serving as Comes sacrarum largitionum (Count of the Sacred Largess) between 416 and 419.

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.