As Beijing's population grows the city government is facing challenges arising from the application of the household registration system.?Du Juan reports.

More than 20 million people live in Beijing, the capital of China and one of the world's great cities. There are many ways to define the city's residents: gender; age; employment status; interests; and personality. One category stands above all others, though — hukou, or household registration, system. Unlike people's personal ID card — similar to a social security number in the United States — which can be used on most occasions to determine a person's identity, the hukou system can affect life at a much deeper level.