China is starting significant reform of one of the most restrictive policies dating back to the Mao era.
The Hukou is an internal passport which classes people as either rural or urban. It's probably the largest social engineering project in the world.
For the rest of their lives people can only go to school, university and settle down in the place they were first registered. Unless they can pay their way out of it.
Based on the old soviet system which separated farm workers from town and city dwellers; the household registration was designed to ensure agricultural production and, to some extent, political stability.
Now China's new leaders are pushing significant new economic reforms which are expected include historic reform of the Hukou system.
China's financial capital, Shanghai, announced this week new rules which would allow people from the countryside to move into the city and enjoy the same benefits, the same access to schools and welfare.
Far above the ground and far from home. Window cleaner Zhao Fei came to Beijing to find work. He's a migrant worker, and so finds himself treated as an immigrant in his own country.
He was born in a rural province, so China's Hukou system prevents him settling in a city.
As he dangles from a rope hanging down the side of a glass clad skyscraper in China's capital, I speak to him through an open window. He tells me state bureaucracy makes him an outsider.
"There's nothing for me in Beijing, I'm like a stranger, a passer-by".
If you don't have a Hukou, you don't have a life. Twenty-year-old Li Xue doesn't officially exist, she's an illegal second child, her parents refused to pay the fine and so she's never had a Hukou: the vital piece of paper registering her with the state.
She couldn't go to school, can't travel, even by train, can't get a job. Despite her dreams of going to university denied; she studies law at home.
In a quiet voice she says she feels lonely, that she always feels different and she's never had any school friends.
For 20 years the family have tried to crawl through the barbed wire of Chinese bureaucracy, the ordeal has taken its toll.
Li Xue's mother, Bai Xiu Ling, cries tears of exasperation. She was sacked for having an illegal second child, then when she couldn't pay the fine, worth five years pay, officials refused to give her daughter a Hukou.
"They never treated us fairly, they're vicious, they never listen to us, it feels like this injustice is killing my daughter" she tells me.
China's Premier Li Keqiang in recent speeches has been enthusiastically pushing for economic reforms which western economists say will be the most modernising changes for decades.
Stripping away red tape, loosening controls over business. Turning more Chinese people from factory workers into consumers.
Wang Tai Yuan is a government advisor and a scholar from Beijing's Public Security University.
"During the Mao era, the old economic system was very rigid, people couldn't move around very easily now we are trying to free people to move around in the new economy" he told me.
The buzzword for China's new leaders, is urbanisation, expanding the cities, liberalising the economy, which will also mean reforming the Hukou system, freeing China's population from its oldest form of state control.