Institutionalist theory analyzes the discrepancy that exists between expressed interests in political behavior and potential interests under different circumstances. Behavioralism has assumed that the political process is perfectly efficient and fair. Historical institutionalist analyses share is the theme of the contingencies of history. Institutions, the mechanisms for aggregating the individual interests, alter and sometimes even shape individual interests. Institutions act as filters that selectively favor particular interpretations of goals or ways of achieving goals and political actors define their interests and shape their strategies depending on the institutional framework of a given society. Historical institutionalism has been able to embrace other "variables" of explanatory power to enhance its own theoretical power. Institutional arguments based on historical contingencies and dynamics of a given era's need to be consistent, indeed explain in many cases the rational principles of individual choice making. The chapter also presents an overview of the key concepts discussed in this book.