This chapter presents a basic overview about psychological theories that are foundational to cooperative learning: social psychology, developmental psychology, cognitive psychology, humanistic psychology, and motivational theories. In so doing, our purpose is to acquaint readers with aspects of theory and research that may be helpful to teachers in understanding the historical development of cooperative learning and its significance to the teaching of a second language. [This article was published in "Cooperative Learning and Second Language Teaching," p9-17 2006.]