This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book delves into the fundamental principles and migration metrics and attempts to comprehend the rapidly changing migratory patterns in relation to time and context. It reviews and synthesizes migration theories, and aims to consolidate and summarize the most commonly debated ideas on migration in the literature, as well as classify the many models and theories into distinct categories for a better understanding of the concerns. The book introduces census-based migration data, both in India and in comparison to other countries. It emphasizes the importance of statistics, particularly in the aftermath of the pandemic. The book examines the various policies and initiatives in India that deal with migration and migrants. It concentrates on various issues of internal migration related to concepts, data, measurement and policy and programmes from the point of view of integrating migration with development.