The causes and mechanisms of the fall of the Western Roman Empire are a historical theme that was introduced by historian Edward Gibbon in his 1776 book The History of the Decline and Fall of the Roman Empire. Though Gibbon was not the first to speculate on why the empire collapsed, he was the first to give a well-researched and well-referenced account of the event, and started an ongoing historiographical discussion about what caused the fall of the Western Roman Empire. The traditional date for the end of the Western Roman Empire is 476 when the last Western Roman Emperor was deposed. Many theories of causality have been explored. In 1984, Alexander Demandt enumerated 210 different theories on why Rome fell, and new theories have since emerged. Gibbon himself explored ideas of internal decline (civil wars, the disintegration of political, economic, military, and other social institutions) and of attacks from outside the empire.

Many historians have postulated reasons for the collapse of the Western Roman Empire. Their conclusions usually belong in two broad schools: (1) external factors, such as military threats and barbarian invasions or (2) internal factors, such as a decline in "civic virtue" and military and economic capability. Most historians believe that the fall was due to a combination of both internal and external factors, but come down more heavily on one or the other as the most important cause of the fall. Modern scholarship has introduced additional factors such as climate change, epidemic diseases, and environmental degradation as important reasons for the decline. Some historians have postulated that the Roman Empire did not fall at all, but that the "decline" was instead a gradual, albeit often violent, transformation into the societies of the Middle Ages.

Comparisons by historians, both professional and amateur, and in literature, both scholarly and popular, of Rome with the decline and fall of other societies have been numerous. "From the eighteenth century onward", historian Glen Bowersock wrote, "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."