Language is a powerful tool for exploring and creating ideas and common knowledge together. Given this, a substantial body of educational research work is concerned with understanding the nature and fostering the efficacy of classroom-based educational dialogues. Classroom talk is not simply a conduit for the sharing of information. Rather, it is the most important educational tool for guiding the development of understanding and for jointly constructing knowledge (Hodgkinson and Mercer 2008; Mercer and Littleton 2007). It is thus vital that educationalists pay careful attention to the quality of classroom talk and induct children and young people into effective ‘ways with words’ such that they become able to use language as a tool to build and explore ideas together. Harnessing the power of talk for reasoning and learning holds the key to children being able to engage sociably and effectively with others, to benefit from reasoned dialogue with their teacher and peers, and ultimately to be empowered as learners with the reasoning capabilities necessary to learn and rise creatively to the challenges posed by an uncertain future (Mercer and Littleton 2007). This emphasis on the significance of social interaction for learning, and language as one of the principal tools for constructing knowledge and understanding, derives from sociocultural theory and has its roots in the seminal work of Lev Vygotsky (see also Vousden, Wood, and Holliman, Chapter 5, this volume).