The number of people in China hit 1.3 billion on 6 January 2005, and it's growing by 10 million every year.
The government thinks the country can't cope with the strain this is putting on society, so it introduced the one child policy to deal with it.
This law only lets couples in cities have one child, unless they are from a minority group, or are both only children themselves.
If they live in the countryside, they are allowed another after a long break.
If people break this law, there can be harsh punishments, and some human rights protestors are against it.