Scholarship on the relationship between domestic institutions and foreign policy is driven by the assumption that a state’s domestic political arrangement can explain important aspects of its foreign policy behavior. Democratic domestic institutions, in particular, are thought to be significant for explaining an important set of outcomes. Research shows, for example, that democracies tend to cooperate with each other; uphold their commitments; make more effective threats; engage in fewer wars with each other (but do fight non-democracies quite frequently); perform better in the wars in which they are involved; and tend to fight wars of shorter duration. Studying the impact of democratic domestic institutions on foreign policy has developed along two broad lines. The first and most established approach is rooted in the basic distinction between democracies and non-democracies. In this view, democratic institutions constrain leaders in a way that produces distinct democratic foreign policy patterns. This approach has yielded a tremendous amount of research and insight into democratic foreign policy, but also suffers from several important shortcomings. One is that democracy tends to be correlated with a host of other variables, making it difficult to specify what exactly it is about democracy that explains certain foreign policy outcomes. A second and related critique of this approach is that it tends to treat democracy uniformly when in fact there is often great variation in democratic domestic institutions across cases. A second and more recent approach focuses on the differences among democracies and seeks to explain how this variation, in turn, creates variation in foreign policy behavior. Democracies differ in terms of their underlying institutional arrangements in a variety of ways, including whether they have presidential or parliamentary systems, autonomous or constrained executives, and open or closed institutions to modulate the flow of information between leaders and citizens, among others. Even within a country, there can be a different set of institutional constraints on democratic leaders depending on the given foreign policy instrument they seek to employ. Studying these variations and their impact on policy processes and outcomes provides great promise for further unpacking the relationship between domestic democratic institutions and foreign policy.