There were several. The most commonly agreed upon reasons are 1) its corrupt, unstable government; the position of emperor was both a corrupt one and a death sentence for its occupant; 2) economic downfall; taxation and inflation were at an all-time high towards the end of the Roman empire, and the empire's commercial/agricultural production declined sharply, lowering resources; 3) invasions of Barbarian tribes; there were many military losses at the hands of Germanic tribes. King Alaric attacked the city and only decades later, the Vandals attacked, and only a couple decades after that Germanic leader Odoacer basically swept up the rest of Rome's glory.