This volume represents the first collection of empirical studies focusing on peer interaction for L2 learning. These studies aim to unveil the impact of mediating variables such as task type, mode of interaction, and social relationships on learners’ interactional behaviors and language development in this unique and pedagogically powerful learning context. To examine these issues, contributors employed quantitative, qualitative, and mixed-methods designs as well as cognitive, social, and sociocognitive theoretical frameworks. The majority of the studies are classroom based and were conducted in a rich array of settings covering five continents and encompassing a wide range of learner L1s and target languages. These settings include second and foreign language classrooms from primary to university level, content-based programs, online contexts, and after-school programs. To span the divide between research and practice, each study includes a section suggesting pedagogical implications.