Immigrant children and adolescents experience intergroup exclusion, which has many adverse psychological and academic outcomes. Bystander challenging reactions are effective in reducing social exclusion in schools. The likelihood of bystander challenging, however, can decrease developmentally. Previous research indicates that group membership, group status, and group norms can affect how youth evaluate, reason about, and react to intergroup bullying. The present thesis extends the existing knowledge by examining how group membership, group status and group norms developmentally influence children’s (aged 8-11 years) and adolescents’ (aged 13-15 years) evaluations of, reasoning about, and bystander reactions to the social exclusion of immigrants and non-immigrants, for the first time, in intergroup compared to intragroup exclusion contexts, drawing from a developmental intergroup approach. Chapter One reviews the literature regarding intergroup exclusion and bystander reactions and outlines the Social Reasoning Developmental model (SRD) upon which this thesis draws. Chapter Two provides a behavioural examination of the role of group membership and group status in how children (8- to 10-year-olds) and adolescents (13- to 15-year-olds, N = 292) react to the intergroup and intragroup exclusion of immigrants and non-immigrants using an online ball-throwing game, Cyberball. In Chapters Three, Four and Five, participants were aged 8 to 10 and 13 to 15 years (N = 340). Chapter Three examines how children’s and adolescents’ evaluation of exclusion and group support change developmentally in intergroup and intragroup peer group contexts. Chapter Four examines the developmental differences in children’s and adolescents’ expectations of peer challenging reactions, and their individual bystander challenging reactions to exclusion in intergroup and intragroup peer group contexts. Chapter Five examines the developmental differences in children’s and adolescents’ indirect bystander challenging reactions to and reasoning about the social exclusion of immigrants and non-immigrants. In Chapters Six and Seven, participants were aged 8 to 11 and 13 to 15 years (N = 463). Chapter Six examines how injunctive peer group norms (i.e., what peers approve of) and descriptive peer group norms (i.e., what peers actually do) influence children’s and adolescents’ bystander reactions to the social exclusion of immigrants and non-immigrants. Chapter Seven examines how injunctive and descriptive peer group norms influence children’s and adolescents’ evaluations of social exclusion and their group’s bystander reactions. Overall, these studies show how group membership, group status, and group norms can play an important role in shaping youth’s decreasing bystander reactions to social exclusion with age. In Chapter Eight, the findings of the current work are discussed in relation to the SRD, and the theoretical, methodological and practical implications are provided.