Population mobility across borders shows a changing landscape of population movements, security, and inter/national policies. This chapter explores the waves of migration in East Asia, focusing on the population inflows and outflows of Japan during the 1920s–2020s. Japan in the mid-2020s accepts the record number of immigrants from abroad; yet, it once experienced the history of emigration to Taiwan, Korea, and other parts of Asia-Pacific. Emigration to the American continents developed until the early 1920s, while Japanese experienced immigration exclusion and discrimination in the Americas. Since then, political and economic changes have shaped the migration trends within and out of East Asia. More recently, the population mobility accelerated and diversified in the age of global migration. Such regional and global transformations have also shaped the migration policy and reality of Japan, previously known as a country of limited population inflows. Apparently, its local economic and population shifts have affected the immigration laws which have been gradually liberalized. In addition, civil movements pushed the country to enact the Hate Speech Elimination Act in 2016. Although Japan was reluctant in accepting refugees, changes appeared that more have received residential status as seen with Ukrainian war evacuees and asylum seekers from Myanmar. Social integration with the currently growing immigrant population is an emerging challenge and social issue of Japan in the first quarter of the twenty-first century.