There was once a time in China when migrant workers were labeled as "blind influx" (mang liu) or "vagrants" (liu min).

When people moved from rural to urban areas in pursuit of a better life, they used to be derogatively depicted as migrant laborers aimlessly struggling for survival and bringing many problems to the cities.

Now, with its migrant population topping 211 million, China takes pride in being a country where the free flow of laborers is preventing stagnation and transforming society.

As indicated in the latest report released by the National Population and Family Planning Commission, migrant population makes up as much as 16 percent of the total, and is expected to double within the next three to four decades.

The far-reaching effects of the successive and unceasing waves of migrants are evident in many aspects of life: They continue to propel the shift in China's growth model, sustain the public interest in social justice and administrative efficiency, and spur the reform of the residential permit (hukou) system.

Despite the wide-ranging impact, the implications of rapid mobility are not always positive.

Fortunately, China's massive migration has not given rise to clusters of slums like in India or Brazil. Nor has it encouraged the emergence of streams of deprived people.

Instead, the pace of migration has always been carefully managed so that it is consistent with China's domestic economic and social conditions. The interplay between policies of urban inclusion and waves of migrant workers has shaped numerous urban miracles that amaze the world.

In the past three decades, the migrant population has expanded 50 times, and millions of laborers have gained more choices to explore their potential, and more opportunities for liberation and development.

China's road ahead, doubtless, will be bumpy.

Its sustainable growth hinges on whether more urban economic centers can be built to attract and absorb more migrants, cope with their needs and deal with the migration-related problems.

Political savvy and vision are needed to tackle the various thorny issues raised by the growing gap between the rate of migration and the pace of infra-structure growth, and laws and regulations.

How can China minimize the negative effects of migration and make it fuel sustainable growth?

How can policymakers address the widening disparity between urban and rural residents, and improve the migrants' quality of life?

There are no easy answers to these questions. Finding feasible solutions will be a defining test of leadership.