As of 2005, the most populated countries according to estimates from the United Nations Population Division are, in descending order, China, India, The United States, Indonesia, Brazil, Pakistan, Bangladesh, The Russian Federation, Nigeria, and Japan.

The Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat estimates that by 2050 world population will approach 9 billion people (currently, world population is around 6.5 billion). Population will increase mostly within developing countries of the world, particularly in urban and coastal areas. Some of the poorest countries in Africa, such as Mali, Niger, Uganda, Burkina Faso, Congo, and Chad are expected to approximately triple the size of their population over the next fifty years. This means that 1/3 or more of these countries' population will be under the age of 15. However, overall, the percent of children is expected to decrease, and populations are expected to "age".

By 2050, India will overtake China as the most populated country (China will still be second), followed by The United States, Pakistan, Indonesia, Nigeria, Bangladesh, Brazil, Ethiopia, and the Democratic Republic of Congo. Issues of sustainability will be crucial to providing food, shelter, and livelihoods to a growing population while maintaining environmental health.

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