Adverse possession in common law, and the related civil law concept of usucaption (also acquisitive prescription or prescriptive acquisition), are legal mechanisms under which a person who does not have legal title to a piece of property, usually real property, may acquire legal ownership based on continuous possession or occupation without the permission (license) of its legal owner.

It is sometimes colloquially described as squatter's rights, a term associated with occupation without legal title during the westward expansion in North America, as occupying real property without permission is central to adverse possession. Some jurisdictions regulate squatting separately from adverse possession.

In general, a property owner has the right to recover possession of their property from unauthorised possessors through legal action such as ejectment. However, many legal systems courts recognize that once someone has occupied property without permission for a significant period of time without the property owner exercising their right to recover their property, not only is the original owner prevented from exercising their right to eject, but a new title to the property "springs up" in the adverse possessor. In effect, the adverse possessor becomes the property's new owner. Over time, legislatures created statutes of limitations setting a time limit for how long owners have to recover possession of their property from adverse possessors. In the United States, for example, these limitation periods vary widely between individual states, ranging from five years to 40 years for real property, or three to five years for personal property.

Although the elements of an adverse possession action differ by jurisdiction, a person claiming adverse possession in a common law system is usually required to prove non-permissive use of the property that is actual, open and notorious, exclusive, adverse and continuous for the statutory period. The possession by a person is not adverse during periods when they are in possession as a tenant or licensee of the legal owner.

Civil Law jurisdictions may recognize a similar right of acquisitive prescription. For example, the French Code Civil 2258 et. seq. recognizes that title may be acquired through thirty years of "continuous and uninterrupted possession which is peaceful, public, unequivocal, and as owner." It is related to the Roman law concept of usucaption or usucapio.

In Denmark, the concept was first mentioned as "Hævd" in Jyske Lov in 1241, though only regulating between peasants and the church, with an asymmetric time limit of 30 years for the church, and 40 years for the peasant. In 1475, the 40 year limit was ruled to apply between farmers as well. In 1547 (after the Reformation) a rule was passed to change this to 20 years for everyone. The rule was later adopted into the Danish Code, published in 1683, this specific part still being in force today. The Norwegian Code from 1688 contains a similar provision.

Personal property, traditionally known as chattel, may also be adversely possessed, but owing to the differences in the nature of real and chattel property, the rules governing such claims are rather more stringent, and favour the legal owner rather than the adverse possessor. Claims for adverse possession of chattel often involve works of art.

In Roman law, usucapio laws allowed someone who was in possession of a good without title to become the lawful proprietor if the original owner did not appear after some time (one or two years), unless the good was obtained illegally (by theft or force). Stemming from Roman law and its successor, the Napoleonic Code generally recognizes two time periods for the acquisition of property: 30 years and some lesser time period, depending on the bona fides of the possessor and the location of the parties involved.^{[citation needed]}