This book focuses on the official regional population projections of Japan and examines its methodologies and results, written in English, by Japanese pioneers in this field for readers who are non-Japanese. In 2018, the National Institute of Population and Social Security Research (IPSS) issued Regional Population Projections for Japan: 2015–2045 (translated title), the only official projection actually released by the Japanese government. Obviously, the population development in the future contains essential information for regional societies. It serves not only for looking at the future of regional societies, but also as a basis for policy plans of local governments, private organizations, and other stakeholders. This present publication incorporates newer findings in the projections methods and the analysis of projection results. Population projection requires looking at three major elements: fertility, mortality, and migration. The migration factor is the most important, so the migration model is given the sharpest focus here. The text explains in an easy-to-understand manner what kinds of assumptions and models were used to produce this work. In addition, it considers the population changes of a future Japanese society in two categories, metropolitan and non-metropolitan areas of the country. At the same time, the book analyzes the demographic mechanisms of population change and aging by prefecture, and the regional distribution and growth patterns of the foreign population by prefecture and municipality.