The argument of this dissertation is that instances of foreign policy change can be best understood as interactions between ongoing dynamics of important aspects of domestic party systems and changes in a state’s normative and material international environment. I identify three types of dynamics of party systems: different patterns of coalition and opposition, different patterns of expression of social cleavages through parties, and redefinitions of the meaning attached to the main axis of competition. These dynamics provide partisan actors with the ideational resources to make sense of changes in the international system, contribute to the creation of new (domestic and foreign) policy preferences and bring about political incentives for the promotion of new foreign policies. The pace, content and fields of change are determined by the specific aspects of a party system undergoing change. Using insights from party systems theory and political sociology, the dissertation promotes the idea that the contestation of foreign policy, the engagement of domestic political actors with developments in the international system, and ultimately foreign policy change, all take place within a thick social and institutional structure that prescribes interests and delineates the terms of debate. In this way, this dissertation introduces in the field of International Relations (IR) and Foreign Policy Analysis (FPA) a view of domestic politics that is made up of constrained but enabled political agents, and social structures that impose continuity while containing opportunities for effecting political change. This is a significant departure from existing works on political parties and foreign policy that usually focus on the partisan effect in government or see parties only as carriers of ideologies or societal preferences. This dissertation applies its theoretical framework to three deep historical case-studies (Willy Brandt’s Ostpolitik, the decision of Canada to enter a Free Trade Agreement with the USA, and Greece’s decision to allow Turkey to acquire the status of an EU candidate-member) and four shorter cases in the shape of a plausibility probe. Using the method of structured-focused comparison, the research shows how, in varying historical, social, institutional and international contexts, foreign policy change was brought about by partisan actors who were constituted by domestic social and institutional structures, but who still found opportunities to engage with these structures and promote their own version of change in accordance with the systemically defined interests of their political parties.