This article aims at defining the concept of “semi-presidential government” and detailing the diversity of its practices. There are in fact three types of semi-presidential regimes: the president can be a mere figurehead, or he may be all-powerful or again he can share his power with parliament. Using four parameters — the content of the constitution, tradition and circumstances, the composition of the parliamentary majority and the position of the president in relation to the majority — the author seeks to explain why similar constitutions are applied in a radically different manner.