The literature on entrenchment as a means to achieve constitutional endurance has grown in recent years, as has the scholarship on unamendable provisions as a mechanism intended to safeguard the constitutional project. However, little attention has been paid to the promise and limits of eternity clauses in transitional settings. Their appeal in this context is great. In an effort to safeguard hard-fought agreements, drafters often declare unamendable what they consider the fundamentals to the political deal: the number of presidential term limits, the commitment to human rights and to democracy, the form of the state (whether republican or monarchical), the territorial integrity of the state, the territorial division of power, secularism or the official religion. This article explores the distinctive role and problems posed by eternity clauses in transitional constitution-building, as guarantees of the pre-constitutional political settlement in such fragile periods. The article also compares unamendability to other techniques of constitution-making in uncertain times, such as sunset clauses, deferring hard choices and other forms of constitutional incrementalism.