The Roman Empire (Latin: IMPERIVM ROMANVM) was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean.

The 500-year-old Roman Republic, which preceded it, had been weakened and subverted through several civil wars. Several events are commonly proposed to mark the transition from Republic to Empire, including Julius Caesar's appointment as perpetual dictator (44 BC), the Battle of Actium (2 September 31 BC), and the Roman Senate's granting to Octavian the honorific Augustus (16 January 27 BC).

Roman expansion began in the days of the Republic, but the Empire reached its greatest extent under Emperor Trajan: during his reign (98 to 117 AD) the Roman Empire controlled approximately 6.5 million km² of land surface. Because of the Empire's vast extent and long endurance, the institutions and culture of Rome had a profound and lasting influence on the development of language, religion, architecture, philosophy, law, and forms of government in the territory it governed, particularly Europe, and by means of European expansionism throughout the modern world.

Marcus Aurelius (Latin: Marcus Aurelius Antoninus Augustus; 26 April 121 – 17 March 180 AD), was Roman Emperor from 161 to 180 AD. He ruled with Lucius Verus as co-emperor from 161 until Verus' death in 169. He was the last of the "Five Good Emperors", and is also considered one of the most important Stoic philosophers. During his reign, the Empire defeated a revitalized Parthian Empire; Aurelius' general Avidius Cassius sacked the capital Ctesiphon in 164. Aurelius fought the Marcomanni, Quadi, and Sarmatians with success during the Marcomannic Wars, but the threat of the Germanic tribes began to represent a troubling reality for the Empire. A revolt in the East led by Avidius Cassius failed to gain momentum and was suppressed immediately.

Marcus Aurelius' Stoic tome Meditations, written in Greek while on campaign between 170 and 180, is still revered as a literary monument to a philosophy of service and duty, describing how to find and preserve equanimity in the midst of conflict by following nature as a source of guidance and inspiration.

Gaius Julius Caesar (Classical Latin: [ˈɡaː.i.ʊs ˈjuː.lɪ.ʊs ˈkaj.sar], July 100 BC – 15 March 44 BC) was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire.

In 60 BC, Caesar, Crassus and Pompey formed a political alliance that was to dominate Roman politics for several years. Their attempts to amass power through populist tactics were opposed by the conservative elite within the Roman Senate^{[citation needed]}, among them Cato the Younger with the frequent support of Cicero. Caesar's conquest of Gaul, completed by 51 BC, extended Rome's territory to the English Channel and the Rhine. Caesar became the first Roman general to cross both when he built a bridge across the Rhine and conducted the first invasion of Britain.

These achievements granted him unmatched military power and threatened to eclipse Pompey's standing. The balance of power was further upset by the death of Crassus in 53 BC. Political realignments in Rome finally led to a standoff between Caesar and Pompey, the latter having taken up the cause of the Senate. Ordered by the Senate to stand trial in Rome for various charges, Caesar marched on Rome with one legion—legio XIII—from Gaul to Italy, crossing the Rubicon in 49 BC. This sparked a civil war from which he emerged as the unrivaled leader of the Roman world.

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.