Research on bystanders' responses to bullying shows the valuable contribution that prosocial or defender behaviors can have in reducing bullying in schools. In this article, we propose that a developmental intergroup approach (i.e., a developing understanding of social identities and related intergroup processes) is required to understand fully when and why children and adolescents help bullied peers in diverse contexts. First, we review theory and evidence on intergroup social exclusion to demonstrate the strength of a developmental intergroup approach when understanding responses to complex social scenarios in childhood and adolescence. Then, we review recent evidence that demonstrates the importance of examining group membership, group identity, and group norms to understand children's and adolescents' responses as bystanders in the context of bias‐based bullying. Finally, we consider implications for school‐based interventions and next steps for research.