Bullying is a social phenomenon that involves more participants than the bully and the victim. Since the identification of other bullying participant behaviors, endeavors have been made to explore what inclines a person to defend a victim of bullying. With the social cognitive theory in mind, the current study examined whether students' level of moral disengagement and defender self-efficacy were associated with engagement in defending behavior with a sample of 573 six to eighth grade students. Additionally, students' perceived relationship qualities with teachers and peers were examined as moderators in the association among moral disengagement, defender self-efficacy, and defending behavior. Results indicated moral disengagement and defender self-efficacy were significantly associated with defending behavior. Student-teacher relationship moderated the association between moral disengagement and defending behavior as well as defender self-efficacy and defending behavior. However, student-student relationship quality did not moderate the relation among these variables. Limitations and implications for future research and practice are discussed. [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.]