From the theoretical perspective of a sociocultural theory of mind, cognition and knowledge are dialogically constructed. Embedded in this theoretical framework, this chapter reviews research studies in which peer-peer dialogue is linked to second language learning as students engage in writing, speaking, listening and reading activities. The review takes the stance that the type of dialogue of particular significance in the language learning process is collaborative dialogue, or that which occurs when learners encounter linguistic problems and attempt to solve them together. In such conditions language is used both as a communicative and cognitive tool. From the studies reviewed, we conclude that peer-peer collaborative dialogue mediates second language learning. We also conclude that although the studies reported few adverse effects of peer collaboration, teaching learners how and why to collaborate may be important to enhance peer-mediated learning. The chapter calls for more research which provides clear evidence of the connections between peer-peer dialogue and second language learning and more studies which investigate longer term learning. Such studies linking peer-peer dialogue to language learning can be instructive for teachers, researchers, and learners, and make us all more aware of the process of learning a second language.