Istanbul is now a megacity with a population of over 12 million. It is the only megacity with a long and at the same time continuous history of more than 2600 years.§The present publication lays stress on the human side of the population and the social aspects of the citizens and their development: The growth of the population; the masses of in-migrants in modern times; the settlements of the poor migrants in shanty towns; their rise from lower class status to middle class people; the expansion of manufacturing and services in Istanbul, making the city the commercial capital of Turkey, raising the incomes of the urbanites and laying the economic basis for a new upper class; the creation of social polarization among the population on the background of effective policies of financial liberalization and globalization today.§Other changes are of importance for the populace: from illiterate and subordinate migrants to literate and politically interested voters; from participants of the bazaar and shopkeeping-type economy to consumers in the American mall-type shopping centers; from pedestrians to users of the various new types of transportation systems as commuters.§Finally, demographic features like changes of birth- and death-rates and family sizes from extended to nuclear family size are part of the population transformation in Istanbul from the pre-industrial level in the early 19th century to the level of industrialized urban society today.