The rule against perpetuities is a legal rule in the common law that prevents people from using legal instruments (usually a deed or a will) to exert control over the ownership of private property for a time long beyond the lives of people living at the time the instrument was written. Specifically, the rule forbids a person from creating future interests (traditionally contingent remainders and executory interests) in property that would vest beyond 21 years after the lifetimes of those living at the time of creation of the interest, often expressed as a "life in being plus twenty-one years". In essence, the rule prevents a person from putting qualifications and criteria in a deed or a will that would continue to affect the ownership of property long after he or she has died, a concept often referred to as control by the "dead hand" or "mortmain".

The basic elements of the rule against perpetuities originated in England in the 17th century and were "crystallized" into a single rule in the 19th century. The rule's classic formulation was given in 1886 by the American legal scholar John Chipman Gray:

No interest is good unless it must vest, if at all, not later than twenty-one years after some life in being at the creation of the interest.

The rule against perpetuities serves a number of purposes. First, English courts have long recognized that allowing owners to attach long-lasting contingencies to their property harms the ability of future generations to freely buy and sell the property, since few people would be willing to buy property that had unresolved issues regarding its ownership hanging over it. Second, judges often had concerns about the dead being able to impose excessive limitations on the ownership and use of property by those still living. For this reason, the rule only allows testators (will-makers) to put contingencies on ownership upon the following generation plus 21 years. Lastly, the rule against perpetuities was sometimes used to prevent very large, possibly aristocratic estates from being kept in one family for more than one or two generations at a time.

The rule also applies to options to acquire property. Often, one of the objectives of delaying the time of vesting is to avoid or reduce taxation of some sort. For example, a bequest in a will may be to one's grandchildren, often with a life interest to one's surviving spouse and then to the children, to avoid the payment of multiple death duties or inheritance taxes on the testator's estate. The rule against perpetuities was one of the devices developed to at least limit this tax avoidance strategy.