The research on predictors and effects of defending victims of peer victimization and bullying continues to grow, but most research on this topic is lacking a strong theoretical framework. This review of defending research introduces interdependence theory as a theory with the capacity to organize many of the empirical findings from the existing defending literature into a meaningful whole. Other theories used to frame defending research are described, and limitations of these theories are discussed. Framing defending research within interdependence theory leads to new research questions. These include: (1) who defends whom and why?, (2) through what processes do individuals in the peer group internalize peer group norms in regard to peer victimization, and how do these internalized norms affect individual perceptions and beliefs that then guide behavior?, and (3) what are the individual and interpersonal factors that affect whether bystanders defend peers, and how does intervention in peer victimization situations affect defenders in addition to victims? Framing defending research within interdependence theory allows for the organization of previous findings and will guide new research.