The 20th century was a century of genocide and other great violence between groups within societies. Already at the beginning of the 21st century, there have been mass killings, civil wars, violent conflict, and terrorism. This article summarizes influences that tend to lead to intense group violence. It then considers prevention, stressing early prevention--and reconciliation as an aspect of prevention--and focusing on central principles and practices. The principles include developing positive orientations to previously devalued groups; healing from past victimization and promoting altruism born of suffering; moderating respect for authority; creating constructive ideologies; promoting understanding of the origins of violence, its impact, and avenues to prevention; promoting truth, justice, and a shared history; and raising inclusively caring, morally courageous children. Practices related to all of these are also discussed. The article stresses the role of progressive change, that is, of psychological, behavioral, and social evolution, in both extreme violence and positive relations between groups; the role of passive bystanders in the unfolding of violence; and the role of active bystandership in the prevention of violence, in the promotion of reconciliation, and in the development of harmonious societies. It emphasizes psychological processes but notes the importance of creating societal institutions. The author cites findings from both laboratory research and case studies, reviews interventions and their evaluation in Rwanda, and points to the need for further research.