The Byzantine Empire or Eastern Roman Empire was the eastern section of the Roman Empire which remained in existence after the fall of the western section. The life of the empire is commonly considered to span AD 395 to 1453.
During the thousand years of its existence, it was known as the
Eastern Roman Empire. It was not referred to as the "Byzantine Empire" until the 17th century.

The Roman emperor Constantine the Great rebuilt Byzantium (today's Istanbul) in AD 330. He renamed it Constantinople and made it the capital of the Roman Empire.

The division of the Empire began with the Tetrarchy (quadrumvirate) in the late 3rd century AD with Diocletian, as an institution intended to efficiently control the vast Roman empire.
The Roman empire was divided by Theodosius I (also called "the great") for his two sons in AD 395. Arcadius became ruler in the East, with his capital in Constantinople, and Flavius Honorius became ruler in the west, with his capital in Milan.

The Byzantines considered to themselves to be Romans and the legitimate continuation of the the Roman Empire. Practically speaking, however, the general prevailing national identity of the Eastern Roman State was Greek. Greek was not only the official language, the language that would represent the Eastern Roman State, but also the language of the church, of the literature and of all commercial transactions. Even though the Byzantine Empire was a multinational state, including Greeks, Armenians, Jews, Egyptians, Syrians, Illyrians, and Slavs, it was considered to be a "Greek state" due to its Orthodox Christian character and its common Greek culture radiated by large centers of Hellenism such as Constantinople, Antioch, Ephesus, Thessalonika and Alexandria.

The Eastern Empire was largely spared the difficulties of the west in the 3rd and 4th centuries, in part because urban culture was better established there and the initial invasions were attracted to the wealth of Rome. In the 6th century the empire under Justinian I even regained some of the lost Roman provinces, conquering much of Italy, north Africa, and Spain. Under his reign, the Hagia Sophia was constructed in the 530s.

Justinian left his successors an empty treasury, however, and they were unable to deal with the sudden appearance of new invaders on all fronts. The Lombards took Italy, the Slavs overwhelmed much of the Balkans, and the Persians gained domination of most of the eastern provinces. These were recovered by the emperor Heraclius, who annihilated the Sassanid kingdom, but the sudden appearance of the Arabs was too much for the empire, and the southern provinces were all overrun in the 7th century.

What the empire lost in territory, though, it made up in uniformity. The southern provinces differed significantly from the northern in culture and practiced monophysite (rather than Orthodox) Christianity, and so felt alienated; the north put up much more of a struggle. By the time of Heraclius the empire had been divided into a system of military provinces called themes to face permanent assault, with urban life declining outside the capital while Constantinople grew to become the largest city in the world. Attempts to conquer Constantinople failed in the face of the Byzantines' superior navy and their monopoly of the still mysterious incendiary weapon Greek fire. After that the empire began to recover.

The empire reached its height under the Macedonian emperors of the 10th and early 11th centuries. Like Rome before it, though, it soon fell into a period of difficulties, caused to a large extent by the growth of the landed aristocracy, which undermined the theme system. Facing its old enemies, the Holy Roman Empire and the Abbasid caliphate, it might have recovered, but around the same time new invaders appeared on the scene who had little reason to respect its reputation - the Normans, who conquered Italy, and the Seljuk Turks, who were mainly interested in defeating Egypt but still made moves into Asia Minor, the main recruiting ground for the Byzantine armies. With the defeat at Manzikert of emperor Romanus IV in 1071 by Alp Arslan, sultan of the Seljuk Turks, most of that province was lost.

The last few centuries of Byzantine life were brought by an usurper, Alexius Comnenus, who began to reestablish an army on the basis of feudal grants (pronoia) and made significant advances against the Seljuk Turks. His plea for western aid brought about the First Crusade, which helped him reclaim Nicaea but soon distanced itself from imperial aid. Later crusades grew increasingly antagonistic. Alexius had granted the city of Venice access to many Byzantine ports for trade. The Venetians became a major threat to the Empire. Under their influence the Fourth Crusade captured Constantinople in 1204, founding a short-lived feudal kingdom and permanently weakening Byzantine power.

Three Byzantine successor states were left - Nicaea, Epirus, and Trebizond. The first managed to reclaim Constantinople in 1261 and defeat Epirus under the Palaeologian dynasty, so reviving the empire but turning attention to Europe when Asia was the primary concern. For a while the empire survived simply because the Muslims were too divided to attack, but eventually the Ottomans overran all but a handful of port cities. Constantinople was initially considered not worth the effort, but with the advent of cannons it fell after a two-year siege to Mehmed II in May 1453. By the end of the century the remaining cities - like Trebizond and Mistra - also fell.

The Byzantine empire played an important role in the transmission of classical knowledge to the Islamic world. Its most lasting influence, though, lies in its church. Early Byzantine missionary work spread Orthodox Christianity to various Slavic peoples, and it is still predominant among them and the Greeks. The start and end dates of the capital's independence, 395 to 1453, were originally the defined bounds of the Middle Ages.

See also Roman Empire, Roman Emperors and Byzantine Emperors.