Reputations are supposed to matter. Decision makers consistently refer to reputations for resolve, and international relations theories confirm the value of being able to credibly signal intentions during times of crisis. However, empirical support for the effects of reputation has been lacking. Problems of strategic selection have hampered previous quantitative tests, and the qualitative literature provides scant support for the concept in individual crises. In this article, the author shifts the focus from crisis behavior to alliance commitments and examines the effects that opportunities to uphold previous commitments have on future alliance commitments and conflicts. The results demonstrate that alliance reputations do affect both alliance formation and dispute behavior. (Contains 20 notes and 4 tables.)