This Article explores the grounds and consequences of illegal occupation. It proposes that an occupation may be considered illegal if it is involves the violation of a peremptory norm of international law that operates erga omnes, and is related to territorial status. Accordingly, illegal occupations are primarily those achieved through violation of the prohibition on the use of force and of the right to self-determination, or maintained in violation of the right to self-determination. This examination forms the basis for a systematic analysis of specific occupations that have been declared illegal by U.N. organs. The second part of the Article addresses the consequences of an occupation's illegality, in view of the political and legal objectives of determining such illegality. It considers the international responsibility for an illegal occupation; the obligation of non-recognition and the law applicable to an illegal occupation; and the right to self-defense. The Article concludes by commenting on the role of “illegal occupation” as a category under international law.