Despite wide recognition of the important role of student bystanders in influencing peer aggression in schools, little is currently known about what influences students to intervene in defense of peer aggression victims. This longitudinal study involving 1,167 primarily white adolescents (aged 12-15 years, 613 females) investigated the role of social cognitive factors and empathy as predictors of students defending victims of peer aggression. High levels of collective efficacy beliefs in the ability of students and teachers to work together to stop peer aggression were associated with higher frequency of defending behavior over time. For girls, empathy was also associated with defending over time. (Contains 2 notes and 4 tables.)