The legitimacy of an emperor's rule depended on his control of the army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or invested with imperial titles by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-Emperors and divide administration of the Empire between them.
The Romans considered the office of emperor to be distinct from that of a king. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, for the first three hundred years of Roman Emperors, from Augustus until Diocletian, a great effort was made to emphasize that the Emperors were the leaders of a Republic.
From Diocletian onwards, emperors ruled in an openly monarchic style and did not preserve the nominal principle of a republic, but the contrast with "kings" was maintained: although the imperial succession was generally hereditary, it was only hereditary if there was a suitable candidate acceptable to the army and the bureaucracy, so the principle of automatic inheritance was not adopted. Elements of the Republican institutional framework (senate, consuls, and magistrates) were preserved until the very end of the Western Empire.
The Eastern (Byzantine) emperors ultimately adopted the title of "Basileus" (βασιλεύς), which had meant king in Greek, but became a title reserved solely for the Roman Emperor and the ruler of the Sasanian Empire. Other kings were then referred to as rēgas.
In addition to their pontifical office, some emperors were given divine status after death. With the eventual hegemony of Christianity, the emperor came to be seen as God's chosen ruler, as well as a special protector and leader of the Christian Church on Earth, although in practice an emperor's authority on Church matters was subject to challenge.
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the west after his forced abdication in 476, although Julius Nepos maintained a claim to the title until his death in 480. Meanwhile, in the east, emperors continued to rule from Constantinople ("New Rome"); these are referred to in modern scholarship as "Byzantine emperor" but they used no such title and called themselves "Emperor of the Romans" (βασιλεύς Ῥωμαίων). Constantine XI Palaiologos was the last Byzantine Roman Emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
Due to the cultural rupture of the Turkish conquest, most western historians treat Constantine XI as the last meaningful claimant to the title Roman Emperor, although from 1453 Ottoman rulers were titled "Caesar of Rome" (Turkish: Kayser-i Rum) until the Ottoman Empire ended in 1922. A Byzantine group of claimant Roman Emperors existed in the Empire of Trebizond until its conquest by the Ottomans in 1461. In western Europe the title of Roman Emperor was revived by Germanic rulers, the "Holy Roman Emperors", in 800, and was used until 1806.