Augustus (Latin: IMPERATOR•CAESAR•DIVI•FILIVS•AVGVSTVS ; September 23 63 BC - August 19 AD 14), born Gaius Octavius Thurinus and prior to 27 BC, known as Gaius Julius Caesar Octavianus (Latin: GAIVS•IVLIVS•CAESAR•OCTAVIANVS ) after adoption, was the first emperor of the Roman Empire, who ruled from 27 BC until his death in AD 14. The young Octavius was adopted by his great uncle, Julius Caesar, and came into his inheritance after Caesar's assassination in 44 BC. In 43 BC, Octavian joined forces with Mark Antony and Marcus Aemilius Lepidus in a military dictatorship known as the Second Triumvirate. As a Triumvir, Octavian effectually ruled Rome and most of its provinces as an autocrat, seizing consular power after the deaths of the consuls Hirtius and Pansa and having himself perpetually re-elected. The Triumvirate was eventually torn apart under the competing ambitions of its rulers: Lepidus was driven into exile, and Antony committed suicide following his defeat at the Battle of Actium by the armies of Octavian in 31 BC.

After the demise of the Second Triumvirate, Octavian restored the outward facade of the Roman Republic, with governmental power vested in the Roman Senate, but in practice retained his autocratic power. It took several years to work out the exact framework by which a formally republican state could be led by a sole ruler, the result of which became known as the Roman Empire. The emperorship was never an office like the Roman dictatorship which Caesar and Sulla had held before him; indeed, he declined it when the Roman populace "entreated him to take on the dictatorship". CCAA, Erich S. Gruen, Augustus and the Making of the Principate, 35. By law, Augustus held a collection of powers granted to him for life by the Senate, including those of tribune of the plebs and censor. He was consul until 23 BC. His substantive power stemmed from financial success and resources gained in conquest, the building of patronage relationships throughout the Empire, the loyalty of many military soldiers and veterans, the authority of the many honors granted by the Senate, Eck, 3. and the respect of the people. Augustus' control over the majority of Rome's legions established an armed threat that could be used against the Senate, allowing him to coerce the Senate's decisions. With his ability to eliminate senatorial opposition by means of arms, the Senate became docile towards his paramount position of leadership.

The rule of Augustus initiated an era of relative peace known as the Pax Romana, or Roman peace. Despite continuous frontier wars, and one year-long civil war over the imperial succession, the Mediterranean world remained at peace for more than two centuries. Augustus expanded the boundaries of the Roman Empire, secured the Empire's borders with client states, and made peace with Parthia through diplomacy. He reformed the Roman system of taxation, developed networks of roads with an official courier system, established a standing army (and a small navy), established the Praetorian Guard, and created official police and fire-fighting forces for Rome. Much of the city was rebuilt under Augustus; and he wrote a record of his own accomplishments, known as the Res Gestae Divi Augusti, which has survived. Upon his death in AD 14, Augustus was declared a god by the Senate, to be worshipped by the Romans. His names Augustus and Caesar were adopted by every subsequent emperor, and the month of Sextilis was officially renamed August in his honour. He was succeeded by his stepson Tiberius.