This chapter defines what represents mutual peer interactions for learning and explains the social and educational relevance of peer interactions, as well as the principles that promote it. It presents the main characteristics of cooperative and collaborative learning. The chapter proposes to take 'cooperative learning' as an umbrella term useful for teachers willing to structure mutual peer interactions in their formal learning in the classroom at primary and secondary schools as well as in higher education. Both cooperative learning and collaborative learning involve high equality between learners. Cooperative learning is rooted in American tradition and anchored in social interdependence, cognitive-developmental and behavioural theories. Collaboration is rooted in British tradition and the linguistic and social nature of knowledge. Learning to cooperate is a core competence for all educational levels, including higher education. Cooperation is a core competence for the knowledge society. So it is recognised by UNESCO itself.