The claim that liberal democracies almost never go to war against each other has been the most studied thesis in international relations in the last decade. A scholarly consensus now seems to hold that democracies are unusually capable of settling disputes. In this elegant restatement of that Kantian vision, Lipson argues that what makes democracies special are their "contracting advantages." In contrast to autocratic states, democracies can make credible, long-term agreements with other democracies, thereby overcoming the uncertainties and insecurities that breed conflict. Lipson identifies various traits that give constitutional democracies this advantage: their openness to outside scrutiny, the continuity of regimes, the electoral incentives for leaders to keep promises, and the constitutional capacity to make enduring commitments. By stressing the deal-making advantages of democracies, Lipson is also able to explain the broader trend toward institutionalized cooperative security and economic relations. His central claim -- that democratic leaders are more constrained by prior commitments and constitutional rules -- is intuitively persuasive but by no means a settled issue, particularly in the new era of U.S. unipolarity.