This dissertation includes three individual articles that explore the relationship between witnessing bullying and defending behavior among students who are bystanders of bullying. Chapter One provides the rationale and purpose of this dissertation and an overview of the three articles that comprise it. Chapter Two employed a cross-sectional analysis to examine the moderating effect of gender on witnessing bullying, defending behavior, and internalizing symptoms while controlling for previous experiences with bullying perpetration and victimization. Results indicated that students who witness bullying and utilize defending behavior exhibit different internal symptoms based on gender. In a longitudinal study, Chapter Three expands this line of inquiry by exploring individual factors that moderate the relationship between gender and defending behavior among student bystanders trained in a brief, bullying bystander intervention (STAC). Results indicated that defending behavior was positively associated with self-esteem for females and fear of negative evaluation for males. To explore the process of defending behavior further, Chapter Four uses a longitudinal study that pairs the Bystander Intervention Model (BIM) with STAC to explore additional factors that may contribute to these differences in student defending behavior. Students trained in STAC reported changes in three of the five steps of the model, with two of the five steps being unique indicators of defending behavior post-training. Chapter Five concludes this dissertation and includes a summary of the three articles. [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.]