This chapter provides the tools—both concepts and theories—used to classify and study groups. It outlines three potential origins: opposition of interest, discrimination and bias against an out-group, and threat. The chapter discusses superordinate goals and intergroup contact. Individual members seek information about social reality and they depend on the majority to validate their understanding of and opinions about the group and the world. Group cohesion refers to the extent to which group members desire to remain in a group and resist leaving it. A highly cohesive group generally maintains a firm hold over its members' time, energy, loyalty, and commitment. Members of groups with high cohesion are more likely to invest both time and energy into their groups because they want them to succeed. When an individual adheres to group norms and standards, it is called conformity. The impact of majority influence on individual group members was illustrated in a series of classic experiments by Asch.