This entry provides a theoretical overview of migration as a psychological phenomenon, outlining key conceptual contributions from both classical and contemporary approaches. It begins by examining early frameworks from social and psychology, including acculturation theory and stress-coping models, and then introduces the contributions of narrative, cultural-historical, and activity-based approaches. These perspectives offer tools to understand identity reconstruction, value negotiation, and meaning-making across cultural contexts. The dialogical self theory, mediated action, and activity theory are highlighted as frameworks that bridge individual experience and sociocultural dynamics. The chapter also considers the role of community psychology and critical perspectives that address power relations, intersectionality, and racialization in migration processes. A variety of methodologies—including quantitative, qualitative, and participatory approaches—are discussed. Finally, the entry reflects on current challenges and debates, such as digital diasporas, anti-immigrant discourses, climate-driven migration, and the need for more inclusive and dynamic theoretical models within psychology.