Every era, every invader, every empire that came and went left its mark in the demographical canvass of Delhi. There has been dramatic movement of population change in every era.

Delhi predominantly had a Hindu population who where mainly traders, who used the Yamuna route to trade with ancient cities like Varanasi and Allahabad, and also administrators as Delhi had always been the seat of power. The demography of Delhi saw a change in its pattern with the Muslim invasion of India during the 12th century. The tyrannical attitude of the Muslim rulers witnessed a gradual change in India's demographic character when a significant part of the Indian population submitted to the invaders by giving up their ancestral faith and embracing Islam. Being the focal point for all invasions Delhi too was caught in the 'change faith, save your skin (and throne)' hubbub.

Another major turning point in demographical canvass of Delhi was during the partition of India in 1947. Delhi, more than other parts of India, witnessed a hue influx of migrants, especially from Sind and Punjab region of Pakistan. Post independence gave Delhi a cosmopolitan character after it became the capital of India and a major business and political nerve center. The city saw a continuous inflow of people from all parts of the country. The economic debacle of some neighboring state also added to rise in Delhi's population.

According to a UN study Delhi's population has more than doubled between 1970 (3.53 million) and 1990 (8.62 million) and seen a rapid increase to make Delhi the seventh most populous metropolis in the world. In 2005 Delhi's population figure stood at 15.3 million. If Delhi's metropolitan area or NCR, which comprises the NCT, the neighboring satellite towns of Noida and Ghaziabad in Uttar Pradesh, and Faridabad and Gurgaon in Haryana, is taken into account, the NCR is the world's sixth most populous with a population of 19.7 million.