Istanbul has existed as an important political, cultural and economic center for almost 2700 years. There are few other cities that can claim such importance for such a long period.
The original city on the site of modern day Istanbul was Byzantion, first established about 650 BC. The city was absorbed into the Macedonian empire when Alexander's father, Philip, conquered the kingdom of Thrace around 350 BC.
When invading armies of European barbarians made life a bit too dangerous in Rome, Emperor Constantine moved his residence in 330 AD from Rome to Byzantion, or Byzantium as the Romans called it. After Constantine's death, the city was renamed Constantinople in his honor.
Constantinople continued as the capital of the Eastern Roman Empire for another 1000 years. While the size of the empire waxed and waned during that period, the city remained a beacon of culture and cosmopolitan living. The Eastern Roman Empire finally dissolved with the capture of the city by forces of the Ottoman Empire in 1453.
Rather than destroy the city, the Ottoman sultans moved in and made Constantinople their capital city. The Ottoman empire continued to exist until 1923, when modern day Turkey was formed. The city was known as Constantinople until 1930, when its name was changed to Istanbul.