This chapter establishes the conceptual, theoretical, and empirical foundations for examining generative artificial intelligence (GenAI) in Teaching English to Speakers of Other Languages (TESOL). It situates GenAI within the historical trajectory of technology integration in language education, from early Computer-Assisted Language Learning (CALL) to contemporary AI-enhanced pedagogies and frames its emergence as a critical moment for TESOL practice. Drawing on recent scholarship and original mixed-methods data from 78 survey respondents and 14 semi-structured interviews with TESOL educators across diverse global contexts, the chapter foregrounds key concerns relating to accuracy, bias, ethics, academic integrity, and over-reliance on automation. To address these issues, it introduces the SECT framework—Social Constructivism, Ethical Use of Technology, Critical Theory, and the Technology Acceptance Model—as the central analytical lens guiding the book, enabling systematic examination of pedagogical affordances, ethical implications, power relations, and teacher adoption. By defining key concepts, outlining the empirical approach, and clarifying the scope and rationale of the book, this chapter establishes a foundation for critically examining the pedagogical, ethical, and professional implications of GenAI in TESOL and sets the agenda for the chapters that follow.