In the literature on civil war onset, several empirical results are not robust or replicable across studies. Studies use different definitions of civil war and analyze different time periods, so readers cannot easily determine if differences in empirical results are due to those factors or if most empirical results are just not robust. The authors apply a methodology for organized specification tests to check the robustness of empirical results. They isolate causes of variation in empirical results by using the same definition of civil war and analyzing the same time period while systematically exploring the sensitivity of eighty-eight variables used to explain civil war in the literature. Several relationships with the onset of civil wars prove robust: large population and low income levels, low rates of economic growth, recent political instability and inconsistent democratic institutions, small military establishments and rough terrain, and war-prone and undemocratic neighbors. Variables representing ethnic difference in the population are robust only in relation to lower level armed conflict. (Contains 3 figures, 4 tables and 24 footnotes.)