罗马帝国不断收到来自北欧的袭击，而罗马最终于476年陷落。西罗马帝国的最后一个罗马皇帝罗慕路斯·奥古斯都向日耳曼首领奥多亚塞投降。英国历史学家爱德华·吉本在《罗马帝国衰亡史》（1776）中认为罗马人堕落了，丧失了公民的美德。吉本说相信死后可享永生的基督教使人懒惰与冷漠，至今仍然如此。格伦·布沃肖克评论说：“从18世纪以后，我们对罗马衰亡耿耿于怀：它被看作是所有已知的衰落的原型，而且因此成为我们担忧自身的象征。”它仍然是最大的历史谜团之一，传统上拥有庞大的研究学者队伍。
另外一些值得注意的时期如378年的阿德里安堡战役，395狄奥多西一世（罗马帝国最后一次政治统一）逝世，406年日耳曼人在罗马军团撤回意大利阻挡亚拉里克一世时跨过莱茵河，408年斯提利科去世，以及随后的西罗马军团瓦解，565年最后一位尝试收复西罗马的查士丁尼一世去世，以及632年伊斯兰教入侵。很多学者坚持认为比起“衰亡”来这些变化更适合被描述成一次复杂的转变。随着时间流逝关于罗马帝国为何衰亡或者是否真的衰亡了提出了很多理论。
The Roman Empire had been repeatedly attacked by invading armies from Northern Europe and in 476, Rome finally fell. Romulus Augustus, the last Emperor of the Western Roman Empire surrendered to the Germanic King Odoacer. British historian Edward Gibbon argued in The Decline and Fall of the Roman Empire (1776) that the Romans had become decadent, they had lost civic virtue. Gibbon said that the adoption of Christianity, meant belief in a better life after death, and therefore made people lazy and indifferent to the present. "From the eighteenth century onward", 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." It remains one of the greatest historical questions, and has a tradition rich in scholarly interest.
Some other notable dates are 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 after the withdrawal of the legions in order to defend Italy against Alaric I, the death of Stilicho in 408, followed by the disintegration of the western legions, the death of Justinian I, the last Roman Emperor who tried to reconquer the west, in 565, and the coming of Islam after 632. Many scholars maintain that rather than a "fall", the changes can more accurately be described as a complex transformation. Over time many theories have been proposed on why the Empire fell, or whether indeed it fell at all.