Co-operative learning is widely accepted as a pedagogical practice that can be employed in classrooms to stimulate students' interest in learning through collaborative interaction with their peers. When children work co-operatively, they learn to listen to what others have to say, give and receive help and discuss different ideas, and in so doing, they learn to develop mutual understandings of the topic at hand. However, whilst co-operative learning provides opportunities for students to dialogue, concern has been expressed about the quality of the discourse that often emerges if students are left to engage in discussions without training in how to interact with others. This chapter discusses the teacher's role in promoting effective small-group discourse. It presents two studies of teachers' discourse during co-operative and small-group learning. These studies provide unique insights into how teachers can use language to promote collaborative dialogue in the classroom during co-operative learning.