In modern historiography, the Western Roman Empire were the Roman Empire's western provinces, collectively, during any period in which they were administered separately from the eastern provinces by a separate, independent imperial court. Particularly during the period from AD 395 to 476, there were separate, coequal courts dividing the governance of the empire into the Western provinces and the Eastern provinces with a distinct imperial succession in the separate courts. The terms Western Roman Empire and Eastern Roman Empire were coined in modern times to describe political entities that were de facto independent; contemporary Romans did not consider the Empire to have been split into two empires but viewed it as a single polity governed by two imperial courts for administrative expediency. The Western Empire collapsed in 476, and the Western imperial court in Ravenna disappeared by 554, at the end of Justinian's Gothic War.

Though there were periods with more than one emperor ruling jointly before, the view that it was impossible for a single emperor to govern the entire Empire was institutionalized by emperor Diocletian following the disastrous civil wars and disintegrations of the Crisis of the Third Century. He introduced the system of the Tetrarchy in 286, with two senior emperors titled Augustus, one in the East and one in the West, each with an appointed subordinate and heir titled Caesar. Though the tetrarchic system would collapse in a matter of years, the East–West administrative division would endure in one form or another over the coming centuries. As such, the unofficial Western Roman Empire would exist intermittently in several periods between the 3rd and 5th centuries. Some emperors, such as Constantine I and Theodosius I, governed, if briefly, as the sole Augustus across the Roman Empire. On the death of Theodosius in 395, the empire was divided between his two infant sons, with Honorius as his successor in the West governing briefly from Mediolanum then from Ravenna, and Arcadius as his successor in the East governing from Constantinople.

In 476, after the Battle of Ravenna, the Roman army in the West suffered defeat at the hands of Odoacer and his Germanic foederati. Odoacer forced the abdication of the emperor Romulus Augustulus and became the first King of Italy. In 480, following the assassination of the previous Western emperor Julius Nepos, the Eastern emperor Zeno dissolved the Western court and proclaimed himself the sole emperor of the Roman Empire. The date of 476 was popularised by the 18th-century British historian Edward Gibbon as a demarcating event for the fall of the Western Roman Empire and is sometimes used to mark the transition from Antiquity to the Middle Ages. Odoacer's Italy and other barbarian kingdoms, many of them representing former Western Roman allies that had been granted lands in return for military assistance, would maintain a pretense of Roman continuity through the continued use of the old Roman administrative systems and nominal subservience to the Eastern Roman court.

In the 6th century, Emperor Justinian I re-imposed direct Imperial rule on large parts of the former Western Roman Empire, including the prosperous regions of North Africa, the ancient Roman heartland of Italy and parts of Hispania. Political instability in the Eastern heartlands, combined with foreign invasions, plague, and religious differences, made efforts to retain control of these territories difficult and they were gradually lost for good. Though the Eastern Empire retained territories in the south of Italy until the eleventh century, the influence that the Empire had over Western Europe had diminished significantly. The papal coronation of the Frankish king Charlemagne as Roman Emperor in 800 marked a new imperial line that would evolve into the Holy Roman Empire, which presented a revival of the Imperial title in Western Europe but was in no meaningful sense an extension of Roman traditions or institutions. The Great Schism of 1054 between the churches of Rome and Constantinople further diminished any authority the emperor in Constantinople could hope to exert in the West.

As the Roman Republic expanded, it reached a point where the central government in Rome could not effectively rule the distant provinces. Communications and transportation were especially problematic given the vast extent of the Empire. News of invasion, revolt, natural disasters, or epidemic outbreak was carried by ship or mounted postal service, often requiring much time to reach Rome and for Rome's orders to be returned and acted upon. Therefore, provincial governors had de facto autonomy in the name of the Roman Republic. Governors had several duties, including the command of armies, handling the taxes of the province and serving as the province's chief houses.

Prior to the establishment of the Empire, the territories of the Roman Republic had been divided in 43 BC among the members of the Second Triumvirate: Mark Antony, Octavian and Marcus Aemilius Lepidus. Antony received the provinces in the East: Achaea, Macedonia and Epirus (roughly modern Greece, Albania and the coast of Croatia), Bithynia, Pontus and Asia (roughly modern Turkey), Syria, Cyprus, and Cyrenaica. These lands had previously been conquered by Alexander the Great; thus, much of the aristocracy was of Greek origin. The whole region, especially the major cities, had been largely assimilated into Greek culture, Greek often serving as the lingua franca.

Octavian obtained the Roman provinces of the West: Italia (modern Italy), Gaul (modern France), Gallia Belgica (parts of modern Belgium, the Netherlands and Luxembourg), and Hispania (modern Spain and Portugal). These lands also included Greek and Carthaginian colonies in the coastal areas, though Celtic tribes such as Gauls and Celtiberians were culturally dominant. Lepidus received the minor province of Africa (roughly modern Tunisia). Octavian soon took Africa from Lepidus, while adding Sicilia (modern Sicily) to his holdings.

Upon the defeat of Mark Antony, a victorious Octavian controlled a united Roman Empire. The Empire featured many distinct cultures; all experienced a gradual Romanization. While the predominantly Greek culture of the East and the predominantly Latin culture of the West functioned effectively as an integrated whole, political and military developments would ultimately realign the Empire along those cultural and linguistic lines. More often than not, Greek and Latin practices (and to some extent the languages themselves) would be combined in fields such as history (e.g., those by Cato the Elder), philosophy and rhetoric.