This chapter represents a slight departure in the book from the treatment of aspects of peer interaction in general, including both child and adult second language (L2) learners. In the previous chapter, we made distinctions in aspects of peer interaction according to age. We now turn our attention to the uniqueness of the child language learner and consider age-related differences in peer interaction and implications for L2 development. Much of recent research on age-related characteristics of peer interaction emanates from the fi eld of educational psychology and predominantly focuses on mainstream classes among monolingual children. However, we have incorporated some of these fi ndings in this chapter where it is relevant to L2 learners in a range of contexts.