In biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything. Demography is used extensively in marketing, which relates to economic units, such as retailers, to potential customers.

For example, a coffee shop that wants to sell to a younger audience looks at the demographics of an area to be able to appeal to this younger audience. According to papers published by the United States Census Bureau, the world population hit 6.5 billion (6,500,000,000) on February 24, 2006. The United Nations Population Fund designated October 12, 1999 as the approximate day on which world population reached 6 billion.

This was about 12 years after world population reached 5 billion in 1987, and 6 years after world population reached 5.5 billion in 1993. However, the population of some countries, such as Nigeria and China is not even known to the nearest million, so there is a considerable margin of error in such estimates. Population growth increased significantly as the Industrial Revolution gathered pace from 1700 onwards.

The last 50 years have seen a yet more rapid increase in the rate of population growth due to medical advances and substantial increases in agricultural productivity, particularly in the period 1960 to 1995 made by the Green Revolution. In 2007 the United Nations Population Division projected that the world's population will likely surpass 10 billion in 2055. In the future, world population has been expected to reach a peak of growth, from there it will decline due to economic reasons, health concerns, land exhaustion and environmental hazards.

There is around an 85% chance that the world's population will stop growing before the end of the century. There is a 60% probability that the world's population will not exceed 10 billion people before 2100, and around a 15% probability that the world's population at the end of the century will be lower than it is today. For different regions, the date and size of the peak population will vary considerably.

Population control is the practice of curtailing population increase, usually by reducing the birth rate. Surviving records from Ancient Greece document the first known examples of population control. These include the colonization movement, which saw Greek outposts being built across the Mediterranean and Black Sea basins to accommodate the excess population of individual states.

Infanticide and abortion were encouraged in some Greek city states in order to keep population down.

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