The 'debate' section of the Review aims at contributing to the reflection on current ethical, legal, or practical controversies around humanitarian issues.The definition of occupation under international humanitarian law (IHL) is rather vague, and IHL instruments provide no clear standard for determining the beginning of occupation. Derived from the wording of Article 42 of the 1907 Hague Regulations, occupation may be defined as the effective control of a foreign territory by hostile armed forces. It is not always easy to determine when an invasion has become an occupation. This raises the question whether or not the law of occupation could already be applied during the invasion phase. In this regard, two main positions are usually put forward in legal literature. Generally it is held that the provisions of occupation law only apply once the elements underpinning the definition setout in Article 42 of the 1907 Hague Regulations are met. However, the so-called 'Pictet theory', as formulated by Jean S. Pictet in the ICRC's Commentary on the Geneva Conventions, proposes that no intermediate phase between invasion and occupation exists and that certain provisions of occupation law already apply during an invasion.The collapse of essential public facilities such as hospitals and water-supply installations, partly due to the large-scale looting and violence that came along with the progress of the coalition forces, in Iraq in 2003 demonstrates that this discussion is not simply a theoretical one. Invading armed forces need clarity as to what rules they need to apply.Three experts in the field of occupation law – Marten Zwanenburg, Michael Bothe, and Marco Sassòli – have agreed to participate in this debate and to defend three approaches. Marten Zwanenburg maintains that for determining when an invasion turns into an occupation the only test is the one set out in Article 42 of the 1907 Hague Regulations, and therefore rejects the 'Pictet theory'. Michael Bothe, while also rejecting the 'Pictet theory', argues that a possible intermediate situation between invasion and occupation, if there is any at all, would be very short and that, once an invader has gained control over a part of an invaded territory, the law of occupation applies. Finally, Marco Sassòli defends the 'Pictet theory' and argues that, in order to avoid legal vacuums, there is no distinction between an invasion phase and an occupation phase for applying the rules of the Fourth Geneva Convention.The debaters have simplified their complex legal reasoning for the sake of clarity and brevity. Readers of the Review should bear in mind that the debaters' actual legal positions may be more nuanced than they may appear in this debate.