Although the relationship between collective efficacy beliefs and team performance has been well-documented, few studies have explored the causal mechanisms that might explain these effects. In the current study, the authors explore the role of backing up behavior, a specific form of teamwork behavior, in explaining why high efficacy beliefs lead to high levels of team performance. Participants were 416 undergraduate business students who were organized into 104 four-person teams. The teams worked on an interdependent, computerized, decision-making task. Results of the study revealed that collective efficacy beliefs were positively related to team performance and that backing up behaviors largely mediated these effects. The theoretical and practical implications of these findings are discussed.