This chapter examines the individual and contextual factors that shape whether bystanders intervene in bullying situations. It highlights the influence of personal characteristics such as gender, age, social skills, empathy, moral reasoning, self-efficacy, and social standing on defending behavior. Contextual influences include peer norms supporting intervention, the victim’s relationship to the bystander, the presence of other defenders, and the broader school climate, particularly regarding social emotional learning. The chapter also addresses the role of teachers as critical adult bystanders, noting that although they are well-positioned to witness bullying, intervention is inconsistent. Collectively, these factors underscore the complexity of defending behavior to illustrate that both individual dispositions and environmental conditions interact to shape bystander decisions.