Do group processes cause violence and aggression? The appropriate answer to the question posed by this chapter is not a straightforward “yes” or “no” but rather “sometimes.” On the one hand, people may be involved in conflicts as a direct consequence of their group membership and with the aim of pursuing the group's interest. Their participation in the conflict and the side they are on is directly determined by their group affiliation. Prominent examples are warfare between nations and hooliganism between rival soccer supporters. On the other hand, there are instances where group processes and social identities are not the immediate cause but nonetheless have an impact on how conflicts are perceived and how they further develop. For example, the intentionality of a group member's negative behavior may be interpreted differently based on stereotypes regarding their group. Relevant theory and empirical evidence for both options will be presented and their practical implications discussed.