The theme of the decline of the Roman Empire was introduced by one of the most influential modern historians, Edward Gibbon, in his widely read The Decline and Fall of the Roman Empire (1776). There is ongoing historiographical debate about what actually happened to the Roman Empire in the 4th–5th centuries. Many theories of causality have been explored and most concern the disintegration of political, economic, military, and other social institutions, in tandem with barbarian invasions and usurpers from within the empire. Gibbon was not the first to speculate on why and when the Empire collapsed. "From the eighteenth century onward," American scholar Glen W. Bowersock has remarked, "we have been obsessed with the fall: it has been valued as an archetype for every perceived decline, and, hence, as a symbol for our own fears."  The story remains one of the greatest historical questions, and has a tradition rich in scholarly interest. In 1984, German professor Alexander Demandt collected 210 different theories on why Rome fell, and new theories have emerged since then.
The decline, seen in retrospect, occurred over a period of four centuries, culminating in the final dissolution of the Western Roman Empire on September 4, 476, when Romulus Augustus, the last Emperor of the Western Roman Empire, was deposed by Odoacer, a Germanic chieftain. Some modern historians question the significance of this date. One reason was that Julius Nepos, the emperor recognized by the East Roman Empire, continued to live in Dalmatia, until he was assassinated in 480. The Ostrogoths who succeeded considered themselves upholders of the direct line of Roman traditions. (The Eastern Roman Empire was going through a different trajectory as it declined steadily after 1000 AD to 1453 with the Fall of Constantinople to the Turks.) Many events after 378 worsened the Western empire's situation. The Battle of Adrianople in 378, the death of Theodosius I in 395 (the last time the Roman Empire was politically unified), the crossing of the Rhine in 406 by Germanic tribes, the execution of Stilicho in 408, the sack of Rome in 410, the death of Constantius III in 421, the death of Aetius in 454, the second sack of Rome in 455, and the death of Majorian in 461 are emphasized by various historians. A recent school of interpretation argues that the concept of "fall" points backward, not forward, and says the great changes can more accurately be described as a complex transformation.
The decline of the Roman Empire is one of the events traditionally marking the end of Classical Antiquity and the beginning of the European Middle Ages. Throughout the 5th century, the Empire's territories in western Europe and northwestern Africa, including Italy, fell to various invading or indigenous peoples in what is sometimes called the Migration period. Although the eastern half still survived with borders essentially intact for several centuries (until the Muslim conquests), the Empire as a whole had initiated major cultural and political transformations since the Crisis of the Third Century, with the shift towards a more openly autocratic and ritualized form of government, the adoption of Christianity as the state religion, and a general rejection of the traditions and values of Classical Antiquity. While traditional historiography emphasized this break with Antiquity by using the term "Byzantine Empire" instead of Roman Empire, recent schools of history offer a more nuanced view, seeing mostly continuity rather than a sharp break. The Empire of Late Antiquity already looked very different from classical Rome.