In international law, terra nullius is territory which belongs to no state. Sovereignty over territory which is terra nullius can be acquired by any state by occupation. According to Oppenheim: "The only territory which can be the object of occupation is that which does not already belong to another state, whether it is uninhabited, or inhabited by persons whose community is not considered to be a state; for individuals may live on as territory without forming themselves into a state proper exercising sovereignty over such territory."
Occupation of terra nullius is one of several ways in which a state can acquire territory under international law. The other means of acquiring territory are conquest, cession by agreement, accretion through the operations of nature, and prescription through the continuous exercise of sovereignty.
A number of modes for acquisition of sovereignty are presently or have historically been recognized in international law as lawful methods by which a state may acquire sovereignty over external territory. The classification of these modes originally derived from Roman property law and from the 15th and 16th century with the development of international law. The modes are: