The main purpose of this study is to investigate the causes of social revolutions. I hypothesize that the three most important factors that explain the onset of social revolution are economic development, regime type, and state ineffectiveness. These variables and revolution, in turn, seem to be mediated by legitimacy. I then discuss the importance of domestic and external factors that trigger social revolutions. Lastly, I provide parsimonious and general models of social revolution, political instability, and political development. There seem to be two implications of these models: first, legitimacy seems to be a crucial variable for prevention of revolution. Second, revolution seems to be a passing phenomenon, which would cease to exist once modernizing or developing countries establish industrial economies and democratic regimes.