Three studies were conducted to examine what factors were associated with expected bystander intervention across two different contexts (i.e., online and offline), two different age groups (i.e., adolescence and young adulthood) and three different forms of victimization (i.e., general cyberbullying, gender-based cyberbullying, and gender-based social exclusion). Prior research exploring bystander intervention has found that bystanders may play a key role in discontinuing victimization. The current research adopts social ecological and social reasoning developmental frameworks to understand when and why bystanders choose to intervene. Given the prevalence of victimization online and off, it is crucial to understand what developmental, individual, social, and contextual factors may influence how bystanders think about victimization as well as the likelihood of intervening to discontinue victimization. The first manuscript explored expected bystander intervention in response to general cyberbullying and examined developmental differences and the influence of personal, family, and societal factors on adolescents' moral judgments and expected bystander responses. Participants included 6th (N = 425, M[subscript ageinyears] = 11.31, SD = 0.62, 49.6% female) and 9th (N = 403, M[subscript ageinyears]= 14.31, SD = 0.52, 47.9% female) grade public school students recruited from three middle schools and two high schools in the Southeastern U.S. Findings demonstrated that higher levels of affective empathy, cognitive empathy, better family management, and higher levels of secure attachment were all positively associated with active bystander intervention in response to cyberbullying. Alternatively, the more experiences of discrimination from teachers that participants reported, the more likely they were to participate in inactive bystander responses. The second manuscript examined young adults' expected bystander intervention in response to gender-based cyberbullying and investigated the role of prior experience with gender discrimination (i.e., as the victim, transgressor, and bystander), and emotional skills and competencies in moral judgments and expected bystander responses to gender-based cyberbullying. Participants included young adults (N = 373, M[subscript age] = 18.89, SD = 0.94, Range= 18-21, 49.6% women, 49.9% men, 0.3% identified as a gender other than men or women, and 0.3% did not disclose their gender identity) recruited through university Psychology courses in the U.S. Participants reported how likely they were to support the victim or bully of gender-based cyberbullying. Results demonstrated that prior experience with gender-based discrimination as a victim was significantly related to supporting the victim behaviors, whereas prior experience as the perpetrator of gender-based discrimination was related to supporting the bully. Further, emotional skills and competencies positively predicted supporting the victim behaviors. The third and final manuscript considered young adults' equitable attitudes, ratings of acceptability, and bystander intervention in response to gender-based social exclusion within Science, Technology, Engineering, and Mathematics (STEM) classrooms. Participants included 199 college students (M[subscript age] = 19.18; Range= 18-25, 121 women and 78 men) in the Southeastern U.S. Participants were randomly assigned to one of two conditions (i.e., female student exclusion or male student exclusion). Findings indicate that acceptability ratings and equitable attitudes predicted bystander intervention. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]