Beijing, China

Travelers throng Beijing's western railway station during the busy holiday period of Chinese New Year on January 20, 2006.

China is the world's most populous nation and is currently experiencing a rapid social transition.

According to a new UN report on urbanization, Beijing is one of 19 existing megacities—cities with populations of ten million or more.

Urban growth has brought economic gain, but it comes at a price. Transportation and traffic issues have plagued Beijing, as have concerns over environmental quality.

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