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.
A significant development has been the rise of the "megacity", conurbations - such as Tokyo, Mexico City, Bombay, Sao Paulo and New York - that have populations in excess of 10 million inhabitants.
In 2000, there were 18 megacities. It is now at 20 and is expected to increase to 22 by 2010.
The pattern of increasing urbanisation is radically different between the developed and the developing world.
In the developed world, urbanisation has been a factor since the industrial revolution and is almost complete. In the US, 80% of the population lives in cities. In the developing world that proportion has been much less until recently.
Analysts fear that in developing countries the provision of houses and basic services will not be able to keep pace with the growing population. One billion people, one-sixth of the world's population, now live in shanty towns.
Developing countries and medium-sized cities are the main contributor to increasing global urbanisation, according to Ms Zlotnik.
"Most of the urbanisation is happening at the lower level. The growth is most rapid in cities of the range half to one million population. It seems that once a region becomes a megacity, it has limited growth potential - there is just not the room available to grow," she said.
"Our surveys and projections indicate that all urban growth over the next 25 years will be in developing countries. In developed countries, urbanisation will remain the same or decline."
The UN team says the milestone of more than half of all humans
living in cities will be reached in the next few months.