This paper reports on a study of peer interaction in ten foreign language (FL) classes, six secondary and four primary, over a period of four months. The focus of this paper is the nature of peer interaction, including the purposes of second language use, and language choice. The data, comprising observation, audio and video recordings of five lessons from each of the classes, and interviews with learners, indicates multiple uses peers make of their time together, and different potential outcomes for learning. The findings suggest second language use varies in purpose and includes both formulaic pattern practice and communication of new information or ideas, and at the same time creates a context for the co-construction of language and a grappling with form-meaning connections in the target language. By exploring peer interaction as a context for second language use and development, this research brings together different perspectives on interaction and second language acquisition and builds on recent calls for a greater awareness of the interdependence of social and cognitive factors in the process of language learning. (Contains 1 figure, 2 tables, 9 excerpts, and 2 endnotes.)