The decline in intergroup relations evident in myriad conflicts around the world has far-reaching implications: it erodes trust and cooperation at both the individual and societal levels, hinders effective societal functioning and threatens the well-being of individuals living in such contexts. In response, researchers have developed evidence-based interventions aimed at improving intergroup relations and cultivating societies that are more inclusive, tolerant and peaceful. However, a ‘one intervention fits all’ approach persists. In this Review, we consolidate research from four domains in social psychology (prejudice reduction, conflict resolution, intergroup reconciliation and affective polarization) to elucidate the critical features necessary for successful intergroup interventions. Specifically, we consider the importance of identifying meaningful intervention goals (what), crucial characteristics of intervention recipients (who) and key contextual features (where) for optimizing interventions. We also describe how motivation and conformity might present barriers to the successful implementation of intergroup interventions in the real world and we suggest ways to overcome these challenges. A thorough understanding of the features that influence intervention outcomes will enable effective personalization and contextualization of existing interventions and development of new ones.