Diocletian became emperor in 284 AD following the death of the Emperor Carus on campaign in uncertain circumstances; Carus's son, Numerian, was then reputedly assassinated, and the army of the East elected Diocletian as his successor. Diocletian blamed the murder of Numerian on his political rival, Aper, who was executed by Diocletian in person for the crime. Diocletian then openly marched against the forces of Carus's other son, Carinus, and defeated and killed him. In 285, Diocletian raised a fellow officer, Maximian, to be his co-emperor, and propaganda images, including those on coins, depict the pair as earthly equivalents of Jupiter and Hercules. In 293 Diocletian created the system of the Tetrarchy 'the rule of four', whereby he controlled the eastern half of the Roman Empire and Maximian controlled the western half; each senior emperor then took a subordinate co-emperor, Constantius for Maximian and Galerius for Diocletian. When the junior emperors had gained enough experience, the senior emperors would retire, and the new senior emperors would chose junior emperors, Severus for Constantius, and Maximin Daia for Galerius. In 305 AD, therefore, Diocletian retired as emperor, the first to do so voluntarily, and forced Maximian to do the same. Maximian returned from retirement, however, while the biological sons of Maximian and Constantius, Maxentius and Constantine resented their exclusion and thus the entire system colapsed while Diocletian was still alive. Diocletian himself died in retirement at his palace in Split in 312 AD.

Beyond the creation of the system of the Tetrarchy, Diocletion is also noted for his reforms concerning the coinage. The silver antoninianus had become so debased that it barely had any traces of silver in it, and was more of a base metal coin, Diocletian therefore issued a new type of silver coin, the argenteus, but left the debased antoniniani with a function in the economy as base radiates, alongside the new follis. He also introduced the gold solidus in 301.

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