This book examines the role of interlocutors and their individual differences (IDs) in second language (L2) development from four theoretical lenses: the cognitive-interactionist approach, sociocultural theory, the variationist approach, and complex dynamic systems theory. A theoretical overview to each approach is written by a preeminent scholar in the framework, and each overview is followed by an empirical study that demonstrates how interlocutor IDs can be fruitfully researched within that framework. To maximize readability and impact, the chapters follow common organizing questions, inviting the engagement of L2 researchers, students, and teachers alike. Collectively, the chapters in the current volume initiate a cohesive discussion of the theoretical roles of the interlocutor within these four popular approaches to SLA; illustrate how interlocutor IDs influence L2 opportunities and/or development; present innovative, original empirical research on interlocutors and their IDs within each approach; and provide theoretical, empirical, and methodological guidance for future research on interlocutors and their IDs. A powerful contribution of this volume, highlighted in the concluding chapter's synthesis, is the common call across all four approaches for the irrefutable role and need for research on interlocutors and their IDs. The volume also demonstrates how, despite theoretical and methodological differences, the four approaches are advancing congruently toward a more robust understanding of the multifaceted and dynamic nature of all interlocutors and their IDs, and thus toward a more complete and accurate picture of their influence on L2 development.