Many emerging democracies across the globe are scrambling to craft new constitutions. The modal constitution being chosen in this most recent wave of democratization is a rather unknown, and under–theorized, type: semi–presidentialism. This article brings semi–presidentialism back to comparative constitutional theory, distinguishing it from presidentialism and parliamentarism, and guarding against its hasty export to new democracies. This article details when, and why, semi–presidentialism can be problematic from the standpoints of democracy, constitutionalism, and the protection of fundamental rights; and the conditions under which it can be supportive of them. After establishing the analytical framework, this article compares developments in two important historical cases of regime change under semi–presidentialism, cases which have also been among the most influential countries for European politics in the twentieth century: the French Fifth Republic and Weimar Germany. The concluding section draws the evidence together.