This chapter focuses on the concept and beginning of military occupation. It first discusses the traditional criteria that must be fulfilled for a territory to be considered occupied: the establishment of effective control over territory by a belligerent power, loss of effective control by its adversary and absence of consent by the latter. Subsequently, those criteria which are not relevant for determining whether a territory is occupied are discussed. On this basis the chapter then addresses the controversy of whether and to what extent the rules of international humanitarian law (IHL) on military occupation already apply during an invasion. Lastly, the chapter deals with special cases, such as occupation without armed resistance and national liberation wars, the relevance of occupation in non-international armed conflicts and the particular legal consequences of a violation of IHL on military occupation.