This dissertation develops a cross-regional theory of party system collapse. The first part of the conditional sequential theory of collapse states that high detachment, low entry barriers, and insider's support are facilitating conditions for the arrival of the outsider. These conditions do not cause the electoral success of the outsider but their conjoint presence makes it more likely. The second part of the argument states that a political system with fiscal centralization and widespread clientelist practices increases the chances of the collapse. It also argues that the high degree of centralization of major traditional parties is a facilitating condition of the collapse. A party system that encounters the conditions proposed in the theory should be more likely to collapse than another party system that does not encounter such conditions. To develop the theory, this dissertation conducts a comparative analysis of the three cases of collapse occurred to date: Italy, Peru, and Venezuela. It also presents a testing of the theory of collapse through the analysis of several cases of non-collapse: Brazil, Colombia, and Ecuador. This study contributes to the existing literature on collapse by offering a clear definition and operationalization of party system collapse and contending that causes of collapse are common across cases and that can be identified following a process-tracing methodology. Also, this dissertation shows evidence that the election of the outsider does not equal to the collapse of the party system. Finally, this study is the first to offer a conditional probabilistic model that explains and predicts the probability of collapse in any democratic system.