Peer interaction is a specific communicative context that occurs when learners work together towards a common goal. It plays a substantial role both in second language classrooms and as a site for research on language learning. However, a large gulf exists between researchers and educators in terms of their conceptualization and use of peer interaction. The purpose of the current volume is to provide second language researchers with teacher insights about peer interaction in language classrooms to address concerns that research is divided from the realities of the language classroom, diminishing its relevance to instructional practice. This book is based on the beliefs and practices of teachers from around the world who responded to surveys, participated in interviews, and allowed us to make observations of their teaching practices. In this first chapter we provide an overview of research on the key benefits of peer interaction, including how it grants learners: unique opportunities for input, output, and for providing and receiving feedback; a place for socialization and the exploration of language identity; support for metacognitive reflection on language use; opportunities to experiment with new linguistic forms and functions; and an avenue for language practice using language in a wide variety of ways.