This paper gives an introduction to cooperative learning (CL), providing a definition of what it is and is not (pseudo-learning groups, traditional classroom learning groups), discussing basic principles, describing two basic types of CL (formal and informal), and listing the benefits of CL suggested by previous research. In order to understand the power of cooperation, it is necessary to understand what is and is not cooperative effort, the types of cooperative learning, the five basic elements that make cooperation work (positive interdependence, individual accountability, face-to-face interaction, social skills, and group processing), and the outcomes that result when cooperation is carefully structured (achievement, psychological health and social competence). While lessons may be structured competitively, individualistically, and cooperatively, cooperation has by far the most powerful and positive influence on instructional outcomes. What makes cooperative learning unique is the quantity and quality of research supporting its use. When efforts are structured cooperatively, there is considerable evidence that students will achieve higher (learn more, use higher level reasoning strategies, build more complete and complex conceptual structures, and retain information learned more accurately), build more supportive and positive relationships (including relationships with diverse individuals), and develop in more healthy ways (psychological health, self-esteem, ability to manage stress and adversity). (Contains 12 references.) (KFT)