What factors explained party system formation in the post-conflict countries of El Salvador, Guatemala, and Nicaragua? The answer to this question is not immediately obvious as the countries present two puzzles for the existing literature on party systems. The first is the surprising path of development that the systems have followed, given the history of the countries and their institutions. The second is the continuity that the three post-transition systems have exhibited. To explain the development of these systems, I propose a path dependent argument that identifies the transition process as a critical juncture (Pierson 2000:75; Collier and Collier 1991:29). The events that took place during the transition, when a competitive and inclusive party system began to take shape, placed these systems on a particular path of development. Early decisions shaped future possibilities and self-reinforcing mechanisms make the probability of departing from the set pattern increasingly unlikely (Pierson 2000:74-6; Collier and Collier 1991:29-31). Chapter One outlines the project's main argument: that the transition process explains party system institutionalization. Chapters Two, on leftist parties, and Three, on conservative parties, analyze how the interactions between four actors (the military, the armed opposition, the conservative political elite, and the economic elite), during the transition determined the degree to which the party system would be polarized and the type of party organizations that would form. Chapter Four explores how polarization and party organization explain party system institutionalization. Polarization increases or decreases the distance between parties, lowering or raising the costs of switching parties from one election to the next, while the type of partisan organization affects the extent to which citizens are bound to the party system, with durable organizations tending to encourage loyalty among voters. Chapter Five explores whether differences across party systems matter for how citizens relate to the party system and democracy. The chapter analyzes how the two party system characteristics that are the focus of the dissertation, polarization and electoral stability, affect voter turnout. The Conclusion discusses the generalizability of the argument and examines some consequences of party system formation for new democracies.