The story of the mixed constitution is the story of the most stable and just constitution. In theory, this is a combination of at least two of the three elementary forms of government (monarchy, aristocracy and democracy), with some advantages that elementary forms may lack. It originated with the deliberation of Greek philosophers, who wanted to draw up a constitution safeguarding against the permanent variation of elementary constitutional forms and against coups d’état. For both Plato and Aristotle, the mixed constitution was, above all, the reflection of a search for balance between the two extreme forms of government, direct (Athenian) democracy on the one hand and the exclusion of the people from governing on the other. The Greek theory was applied by the historian Polybius to the traditional tripartite constitution of the Roman republic. In his view, the consuls were monarchic elements, the senate an aristocratic element, and the comitia a democratic one. Cicero’s introduction of the idea of the mixed constitution in De re publica can only be understood in the light of the author’s personal situation and contemporary political circumstances. His political engagement at a time when the republic was gradually transforming into a monarchy aimed at restoring the important role of the nobility, represented by the senate. For Cicero, the mixed constitution was mainly an instrument for restoring the lost balance between the consuls, the senate, and the comitia, a last chance to save the decaying republic. The concluding part of the article addresses Alois Riklin’s recent discussion of the modern reception of the mixed constitution idea, which advances the controversial thesis that the paradigm of power division, the foundation of modern representative democracy, originates directly from the mixed constitution.