This is an interdisciplinary textbook on internal and international migration. It is designed to provide comprehensive information on these phenomena to senior and graduate students who have had a basic course in statistics. It is intended to be the sole or supplementary course book for migration courses to students of departments of demography, economics, sociology, anthropology, political science, geography, environmental studies, and public policy. It is based on the experience of developing such a course at the University of Chicago and teaching it for five years. At least a quarter of the materials presented are based on original research by the author. It seeks to reconcile and integrate theories and empirical findings with fundamental principles of the social sciences, rather than attempting to set up separatist or unique explanations. One consequence is to conclude that there is no "true" solution to today's migration problems and their proper "reform"; solutions will depend upon the interaction (often confrontational) of multiple forces of migrants and natives. Its aim is to provide a better understanding of what is happening, and why, while confessing that the outcome is uncertain. History has tended for squatters' (migrants') rights eventually to reach a tolerable compromise with the native social system, which also gets substantially changed by the interaction. This book documents that environmental differences are powerful forces in determining the eventual compromise.