Jurisprudence distinguishes between mandatory rules (which parties are obliged to follow, and hence which apply in all cases) and default rules (which parties are free to override, and hence which apply only in the event that parties fail to opt out of them). This article develops a theoretical framework for the construction of default rules within inheritance law. Its foundational claim is that the modern theory of default rules developed within transactional law provides a model broadly, if not perfectly, applicable to inheritance defaults. Transactional default rule theory suggests that by replicating the hypothetical bargain of a majority of parties, lawmakers can minimize transaction costs. If default rules in inheritance law correspond, by analogy, with the probable intent of a majority (or plurality) of benefactors, lawmakers can achieve the same efficiency. The article looks at, and rejects, other suggested approaches to the formulation of inheritance defaults, which have focused on furthering state interests or on fulfilling an expressive function of law. After setting out a basic theory of inheritance defaults, and comparing it to competitors, the article proceeds to address how inheritance defaults should be structured, and how lawmakers should go about discovering benefactors’ probable intent. A final section surveys existing inheritance defaults, identifying ones that surely or likely fail to effectuate probable intent, and hence which merit substantive revision.