In contrast to the work on presidentialism and parliamentarism, semi-presidentialism remains very much the poor relation in the debate about regime types. This is true both in the sense that there is less work on semi-presidential regimes and also because of the fact that semi-presidentialism has few advocates. This review examines the existing work on semi-presidentialism and asks three questions: What is semi-presidentialism? What is the main dependent variable in semi-presidential studies? And what is the most appropriate explanatory variable in such studies? It does provide some answers to these questions, but the main purpose is to highlight some of the most problematic issues in the contemporary study of semi-presidentialism.