A measure of collective efficacy was developed and administered to undergraduates working in project teams in engineering courses. Findings in each of two samples revealed that the measure contained a single factor and was related to ratings of team cohesion and personal efficacy. Collective efficacy was also found to relate to indicators of team performance at both individual and group levels of analysis. Consistent with social cognitive theory, collective efficacy was a stronger predictor of team performance than team members' perceptions of their self-efficacy. We consider the implications of these findings for further research, theory, and practice on team functioning within occupational and educational settings.