The global population is on the move. By some estimates, 175 million people (nearly 3% of the world population) are migrants, or people who live somewhere other than their birth country. Access to information, improved transportation and economic opportunity has increased the migrant population, a trend expected to continue over the next fifty years. Although the traditional practice of male migrants working and sending money to families at home still exists, women now make up nearly 50% percent of migrants. For both men and women, new migrants often find work in their new destination plagued by the three Ds--dirty, difficult and dangerous.

Many factors stimulate migration from home countries--overpopulation, few economic opportunities, degraded environments and political unrest. However, just as many factors draw people to new locales. Perceptions of political freedom, better jobs and family connections often bring migrants to new places. As a result of this migration, many places--especially rural and poorer settings--suffer from brain-drain, which occurs when an area's best and brightest leave to seek education or economic opportunities elsewhere. Many times, these individuals never return. Although many migrants mail money home, the communities lose their most powerful resources for improvement, their best and brightest citizens.

Migration within developed countries has distinctively different dynamics than that of developing countries. With large numbers of people in urban areas, internal migration often flows outward from the city, reducing urbanization rates and spreading populations across greater distances. Between 1996 and 2001, Toronto, Canada saw 258,000 people moving out of the city and only 214,000 Canadians moving in. Canadian officials attribute this outward migration--which also affects Montreal and Vancouver--to the retirement of an aging population. Financially comfortable but desiring more affordable housing, Canada's retirees are moving to smaller towns to enjoy a slower lifestyle. Far from an isolated trend, Great Britain and the Northeastern United States also experienced similar population declines in many of their central cities.

Although it's not always welcomed by host country citizens and can cause social disruption, inward migration fills critical economic needs. In Japan, immigration may play a key role in sustaining the economic foundation of its cities.   Like other highly developed countries, Japan's workforce has declined since 1998 as its population rapidly ages and fertility drops. Subsequently, workers are scarce, the nation's tax base is thinning and its pension system is more strained by the year. The United Nations estimates that Japan needs 400,000 new immigrants every year to balance the decline. While many come from nearby Asian countries, some come from countries as far away as Peru and Bolivia. The job market for migrants is tough, however. Like many other developed countries, immigrants take the work many young Japanese don't want--the dirty, difficult and dangerous jobs.

Migration in developing countries is a different matter altogether. In China, the migrant population is a staggering 94 million, due to people who move to the more prosperous provinces. Yiwu, a new city in the Zhejiang province, has seen rapid growth in the past two decades. Today, its 500,000 person migrant population is nearing the official resident population of 600,000. In 2000, China limited population mobility by relaxing its hukou system where migration is restricted through residence permits. As a result, millions of former farm laborers relocated to the eastern coastal regions in search of the factory work coming with China's trade liberalization and increased foreign investment. Like elsewhere, Chinese migrants often find themselves working in the lower-paying, dirty and more dangerous jobs local residents try to avoid. The migrants' semi-legal status makes them susceptible to exploitation and discrimination.