Despite the fact that policy-makers and scholars of international politics have often expressed skepticism about the efficacy of international agreements, formal military alliances have been an important feature of international politics for centuries. This essay first introduces the dominant explanation for why states sign alliance treaties: state leaders use formal alliances to convey credible information about their future intentions to partners and adversaries. It then considers empirical evidence in support of this perspective, particularly with regard to deterrence and compliance. Following this summary, I raise five puzzles that contemporary researchers are working to resolve, but which leave room for further analysis and development. First, I discuss the challenges faced by large n empirical studies of alliance formation. Next, I ask why strong states ally with weak states and consider some of the most compelling recent explanations. Third, I consider the complicated relationships between alliances and war. Fourth, I examine how alliances affect cooperation among member states. Finally, I encourage scholars to continue a recent focus on how alliances are designed. While we have seen significant progress in understanding military alliances over the last 20 years, primarily because of the development of game-theoretic models that capture strategic interaction and the collection of new data that allow for nuanced tests of the hypothesized relationships, there is good reason to believe that we will continue to see significant innovation over the next decade.