What makes international threats credible? Recent theories point to domestic audience costs—the domestic price a leader would pay for making foreign threats and then backing down. This article provides the first direct evidence of audience costs. The analysis, based on experiments embedded in public opinion surveys, shows that audience costs exist across a wide range of conditions and increase with the level of escalation. The costs are evident throughout the population, and especially among politically active citizens who have the greatest potential to shape government policy. Finally, preliminary evidence suggests that audience costs arise because citizens care about the international reputation of the country or leader. These findings help identify how, and under what conditions, domestic audiences make commitments credible. At the same time, they demonstrate the promise of using experiments to answer previously intractable questions in the field of international relations.I thank Time-sharing Experiments for the Social Sciences (TESS), the National Science Foundation (CAREER Grant SES-0548285), and the Center for Advanced Study in the Behavioral Sciences for financial support. Colleagues at Knowledge Networks provided invaluable assistance in fielding the surveys. For helpful comments I am grateful to Jim Fearon, Page Fortna, John Freeman, Jon Krosnick, Skip Lupia, Helen Milner, Diana Mutz, Ken Scheve, Ken Schultz, Jas Sekhon, Alastair Smith, Paul Sniderman, Rob Van Houweling, Jonathan Wand, Jessica Weeks, and the anonymous referees. I also thank seminar participants at Berkeley, CASBS, Columbia, Duke, NYU, Rice, and Yale, and conference participations at the International Studies Association and the TESS meetings at the University of Pennsylvania.