This article lays out a theoretical framework for understanding the effects of political uncertainty on party development and strategies of mobilization and competition. Defining uncertainty as the imprecision with which political actors are able to predict future interactions, the authors identify three types of political uncertainty: regime uncertainty, economic uncertainty, and institutional uncertainty. They argue that political uncertainty is particularly high among developing democracies, contributing to puzzling empirical patterns of party development and competition in these contexts. Taking into account the role of uncertainty in the strategic decision making of party elites will help scholars better understand the differences between parties in advanced and developing democracies. But it can also help scholars understand the less dramatic differences between parties even within advanced democracies. The authors’ theoretical framework can be applied broadly since uncertainty informs the strategic choices of a much wider range of political actors.