By David Whitehouse
Science editor, BBC News website
More than half of all humans will soon be living in cities, according to a prediction by the United Nations.
The developing world is a major contributor to urbanisation
"Psychologically it is an important step for mankind," Hania Zlotnik, director of the United Nations Population Division, told the BBC.
There are concerns that, in developing countries, basic provisions in cities will lag behind population growth.
Observers will see increased pressure placed on resources and services as humankind becomes an urban species.
"It's an increasing trend that is becoming more obvious. People do not realise how rural the world was until recently. That is changing," Zlotnik said.
Despite almost four millennia as centres of civilisation, it was only fairly recently that cities attracted more than a small percentage of the global population. With hindsight, the 20th Century was the century of urbanisation.
In 1900, only 14% of humanity lived in cities. By the century's close, 47% of us did so. This change is revealed in the growth of the number of medium-sized cities. In 1950, there were 83 cities with populations exceeding one million; but by 2000, this had risen to 411.
It is in cities that tackling pollution will be paramount. In them, the adequacy of political institutions will be tested as well as the ability to provide city dwellers with basic facilities.
While the world's urban population was just one billion in 1804, by 1985 it had risen to two billion and by 2002 it was three billion. If the trend continues, the world's urban population will double every 38 years, say researchers.