From the early 1920s, the city was extensively modernised on a grid system, and this period marked the start of phenomenal population growth and uncontrolled urban development. An educated and cosmopolitan middle-class elite, with an open attitude towards Western influence, flourished under the Shah, but the growth of the city started to attract a slow trickle of poor, rural migrants that soon turned into a flood.

The depopulation of the surrounding regions continues to this day as the rural poor continue to stream into Tehran in ever greater numbers. This migration has put the city's infrastructure under enormous pressure. In 1930 the population was 300,000; in 2001 it was estimated to be 12 million. Such pressures can translate into popular revolt and Tehran was the hub of activity during the Islamic Revolution of 1979. More recently, protests against the clerical monopolisation of political power have begun in Tehran before spreading to other Iranian cities.