The discovery doctrine, also called doctrine of discovery, is a concept of public international law expounded by the United States Supreme Court in a series of decisions, most notably Johnson v M'Intosh in 1823. Chief Justice John Marshall explained and applied the way that colonial powers laid claim to lands belonging to foreign sovereign nations during the Age of Discovery. Under it, title to lands lay with the government whose subjects travelled to and occupied a territory whose inhabitants were not subjects of a European Christian monarch. The doctrine has been primarily used to support decisions invalidating or ignoring aboriginal possession of land in favor of modern colonial/imperial governments, such as in the 2005 case of Sherrill v Oneida Nation.

The 1823 case was the result of collusive lawsuits where land speculators worked together to make claims to achieve a desired result. John Marshall explained the Court's reasoning. The decision has been the subject of a number of law review articles and has come under increased scrutiny by modern legal theorists.

Accession has different definitions depending upon its application.

In property law, it is a mode of acquiring property that involves the addition of value to property through labor or the addition of new materials. For example, a person who owns a property on a river delta also takes ownership of any additional land that builds up along the riverbank due to natural deposits or man-made deposits.

In commercial law, accession includes goods that are physically united with other goods in such a manner that the identity of the original goods is not lost. In English common law, the added value belongs to the original property's owner. For example, if the buyer of a car has parts added or replaced and the buyer then fails to make scheduled payments and the car is repossessed, the buyer has no right to the new parts because they have become a part of the whole car.

In modern common law, if the property owner allows the accession through bad faith, the adder of value is entitled to damages or title to the property. If the individual who adds value to the owner's chattel (personal property) is a trespasser or does so in bad faith, the owner retains title and the trespasser cannot recover labor or materials. The owner of the chattel may seek conversion damages for the value of the original materials plus any consequential damages. Alternatively, the owner may seek replevin (return of the chattel). However, the owner may be limited to damages if the property has changed its nature by accession. For example, if a finder discovers a gemstone and in good faith believes it to be abandoned and then cuts it and integrates it into a work of art, the true owner may be limited to recovery of damages for the value of the gemstone but not of the final art piece by way of replevin. The remedies and application of the law vary by legal jurisdiction.