This chapter examines classrooms as a social context where teachers use language as a communicative system to facilitate learners’ thinking and learning, while learners use language to demonstrate their thinking and understanding. It draws on sociocultural perspectives of learning, and views language as not only a social tool to communicate but also a cognitive tool to process and manage meaning. Using sample classroom interactions from English language lessons as well as lessons of other curriculum subjects where English is used as a medium of instruction, the chapter explores patterns of language use found in teacher’s talk and learners’ talk during lessons. Specifically, it focuses on features and functions of teacher’s questions, scaffold/feedback, and instructions given during lesson transitions from whole-class teaching to group work. The chapter aims to provide insights into the important roles teacher’s discourse plays in promoting learners’ active engagement during lessons and facilitating their thinking.