Population numbers change over time, influenced by levels of births, deaths and migration into or out of the population area. Global population levels, having grown slowly for most of human history, are now rising very fast.

Population pyramids show the structure of a population, comparing relative numbers of people in different age groups. Population structures differ markedly between LEDCsLEDC: A Less Economically Developed Country [LEDC] has low levels of development, based on economic indicators, such as gross domestic product (the country's income). More of the countries in the southern hemisphere (eg countries in southern Africa, southern Asia and South America) are LEDCs, while more of the countries in the northern hemisphere are More Economically Developed Countries [MEDCs]. and MEDCsMEDC: A More Economically Developed Country [MEDC] has high levels of development, based on economic indicators, such as gross domestic product (the country's income). More of the countries in the northern hemisphere are MEDCs, eg UK, USA, Canada, Europe. More of the countries in the southern hemisphere are Less Economically Developed Countries [LEDCs]..

Demographic transition models show population change over time - and again show marked differences between LEDCs and MEDCs.