The massive movement of people in these huge Asian countries is leading to important transformations of their regional demographic balance as well as of the age structure of populations in the regions. The net movement is primarily from poorer to wealthier regions, and particularly to the dominant cities of these regions, where employment opportunities are perceived to await the newcomer. Thus the share of national populations in the lagging regions is declining, despite their frequently higher fertility rates. This synthesis chapter discusses some of the measurement issues related to migration analysis as methodological and definitional challenges come to take a more important place than is the case for the analysis of mortality or fertility. The chapter also stresses the mounting role of migration in modifying the patterns of population distribution as well as the population structure as fertility rates drop to well-below replacement level (China) and to near-replacement level (India and Indonesia). What is also clear is the symbiotic relationship between migration and urbanization. Whereas in much of Southeast Asia, half a century ago, migration was still largely a process of moving to agricultural frontier areas, currently the dominant migration flows are from rural areas to cities or from one urban area to another and urbanization is clearly increasing in all three countries. Migration will continue to re-shape the populations of Asian countries, particularly those with half or more of their populations currently living in urban areas. Though China and Indonesia have both passed the 50 % urban landmark, there is still enormous potential for growth of their urban populations through migration, and even more so in India, where urbanization has not progressed as far.