Military alliances are one of the most important tools states can use to counter international threats, exert influence over other countries, and accomplish broad foreign policy goals, including peace and stability in the international system. Alliance scholarship can provide valuable insights to policy-makers by answering questions such as which states are likely to ally, when alliances will be upheld, and whether they will be effective. Traditional alliance research focused on international considerations rather than domestic politics as drivers of alliance politics. More recently, however, scholars have shifted their attention to uncovering the ways in which domestic political institutions affect alliance behavior. The main questions underlying this research include: Are states with similar regime types, especially democracies, more likely to ally? Are democracies more reliable alliance partners? Do wartime coalitions involving democracies have a higher chance of victory? Do domestic institutional changes affect alliance maintenance? While the literature has provided conclusive answers to some of these questions, others are characterized by mixed findings. Recently published work has taken on unresolved issues and provided new and original insights. Future research should take these efforts further by unpacking the concepts of “domestic politics” and “alliance politics”.