Three studies were to conducted to examine the correlates of adolescents' bystander responses to different forms of bullying in schools, namely generalized (based on individual differences such as being shy) and bias-based bullying (due to group membership such as being an immigrant or a refugee). Research on bystanders' responses to bullying demonstrates that prosocial or defender bystander behaviors play a role in reducing bullying in schools. The research presented adopts both a developmental intergroup and a social-ecological framework to understand when and why adolescents help bullied peers in different social contexts. The relations between different forms of bullying, social-cognitive development, intergroup relations, and their manifestations are multifaceted and warrant studies to inform the efforts to tackle the pervasive problems of different forms of bullying. The first manuscript investigated factors related to bystander intervention and victim retaliation in response to different types of peer aggression and examined the relationships between responses to aggression and social-emotional factors. Participants consisted of 6th and 9th graders in the USA ("Mage" = 13.27 years, SD = 1.77, N = 896, 52.8% female), who reported whether they would intervene if they witness bullying and following retaliation. Findings documented that effortful control and justice sensitivity (observer) predicted acceptability judgments regarding bystander intervention. In a similar line, participants' higher levels of affective empathy, justice sensitivity as observer and sympathy were positively related to participants' likelihood of engaging in active bystander responses. On the other hand, participants with higher rejection sensitivity and negative affect were more likely report that they would show inactive responses to bullying. The second manuscript investigated whether social-cognition and intergroup processes factors shape adolescents' judgments and responses to bias-based and generalized bullying. Participants included 179 6th ("Mage" = 11.83 years, N = 96, 60 female) and 9th ("Mage" = 14.64 years, N = 83, 48 female) students in the USA. Participants rated how likely they would intervene if they witnessed generalized bullying of nonimmigrant and bias-based bullying of immigrant peer. Results demonstrated that nonimmigrant-origin adolescents reported that they expect they would be less likely to intervene in bias-based bullying. Further, intergroup contact and Theory of Mind (ToM) positively predicted active bystander responses. The third manuscript examined 587 Turkish adolescents' ("Mage" = 13.14 years, SD = 1.61) bystander responses towards generalized (when Turkish youth are bullied) and intergroup (when Syrian refugee youth are bullied) in a within-subjects design. Adolescents read two hypothetical stories with either ingroup or outgroup targets of the bullying. After each story, adolescents evaluated the acceptability of bullying and the likelihood of different types of bystander responses. Findings revealed that adolescents judged intergroup bullying as more acceptable and were more likely to explicitly support the bully in bias-based bullying compared to intragroup bullying. Results also showed that adolescents with higher ToM and empathy evaluated intergroup bullying as less acceptable and were more likely to expect that they would challenge bullying. Further, adolescents with more prejudicial attitudes and discrimination were more likely to see intergroup bullying as acceptable and more likely to support the bully. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]