China's rural to urban migration and urbanization is one of the major stories of of the 21st Century. If current estimates are to be believed some 250 million Chinese have moved from their rural homes to urban areas. This is a scale that the world hasn't seen. Whether this migration fits with classic theorizations of migration, for instance Lee (1966) or du Toit (1990), as the scale far exceeds the cases given in their work. Additionally the migration in China is primarily internal, but it has specific processes and migratory barriers, particularly the Hukou or Household Registration System (户口), which make the Chinese case unique. 

Set up in 1958 this system divides the population into roughly two types of registration, rural and urban, and one's social services are tied to where you are registered. Registration among the rural populations is not fluid, and it is difficult to gain urban residency if you are from another province. The Hukou effectively acts as an internal passport system, with invisible walls (Chan 2008). This hasn't discouraged migration, but creates complex systems of movement, which are tied both to regional inequalities (the Eastern productive coast vs the rural hinterland in the center and west) and to labor. 

This system creates a mobile labor source in China, upon which its vast wealth accumulation since reform and opening has been built. This system was not originally designed to punish the rural areas, where Communist power was initially based, instead designed to create a farmer state. . . but in the 1980s, and especially in the Neo Liberal 1990s, these barriers were used to develop one part of the country before the other, utilizing the labor power of rural masses. 

This is just a rough overview of the general context of migration in China. Reforms in the past years, particularly since 2008, are changing the situation, but the literature is still catching up on what exactly is happening. What the outside observer needs to know is that in China's urban centers like Beijing there are many migrants (sometimes called the Floating Population), working at the lower rungs of society, as factory workers, service workers, and so on. They do not have the same rights as urban residents, with difficulty obtaining housing, health care, and education. In Beijing it is estimated that there are around 8 million rural migrants in the population of 21 million (or more). . . 

These migrants often seem invisible, but they are everywhere. When I was teaching in Beijing, one of my assignments was for the students (both Korean and Chinese) to find someone born outside of Beijing, who didn't have BJ hukou, and to observe them and if possible interview them about their life story. This was always illuminating for the students, who admitted they hadn't thought much about where these people had come from. 

In 2010's Spring Festival I followed a family friend who was a migrant worker back to his village in Anhui province. The above pictures come from this trip, packed on buses moving through the countryside. Spring Festival is always a time of massive population movement, with many migrant workers returning home for the only time in the year. This incredible Baidu data visualization project maps the migratory patterns from all China during 2013's Spring Festival.  

A key anthropological study of migrant enclave areas, not-quite slum neighborhood where migrants from one particular province or regional area congregate, is Li Zhang's Strangers in the City: Reconfigurations of Space, Power, and Social Networks within China's Floating Population. Leslie T. Chang's book Factory Girls: From Village to City in a Changing China, is another major popular text on the topic. Many other books deal with various related topics to this issue, from migrant education to the now growing number of Chinese migrants heading to Africa. 

I would argue that China's global cities, Beijing, Shanghai, Guangzhou/Shenzhen, have been built upon the backs and labor of China's rural populations. They benefit from this growth, but not as much as the relatively rich urban populations. How China integrates these people into urban environments and within the networks of social services, particularly as China is beginning to face numerous forms of scarcity of resources (see Ma & Adam's, 2012 for a detailed discussion of these issues).