This introductory chapter charts the evolving theoretical, methodological, and contextual landscapes of language teacher agency. It reexamines core definitions, clarifies distinctions between agency and related concepts such as autonomy, and highlights its dynamic, relational, and ideological nature. Drawing on ecological, sociocultural, and critical perspectives, the chapter presents key empirical insights that illuminate how agency emerges through interactions with identity, affect, power, and policy across varied educational settings. It also showcases methodological advancements that capture the layered and situated enactments of agency in real-world classrooms. Ultimately, the chapter introduces a praxis-oriented, transperspectival approach that seeks to inform future research, policy development, and transformative practice in global language teacher education and professional development.