Cyber warfare figures prominently on the agenda of policymakers and military leaders around the world. New units to ensure cyber security are created at various levels of government, including in the armed forces. But cyber operations in armed conflict situations could have potentially very serious consequences, in particular when their effect is not limited to the data of the targeted computer system or computer. Indeed, cyber operations are usually intended to have an effect in the 'real world'. For instance, by tampering with the supporting computer systems, one can manipulate an enemy's air traffic control systems, oil pipeline flow systems, or nuclear plants. The potential humanitarian impact of some cyber operations on the civilian population is enormous. It is therefore important to discuss the rules of international humanitarian law (IHL) that govern such operations because one of the main objectives of this body of law is to protect the civilian population from the effects of warfare.This article seeks to address some of the questions that arise when applying IHL – a body of law that was drafted with traditional kinetic warfare in mind – to cyber technology. The first question is: when is cyber war really war in the sense of 'armed conflict'? After discussing this question, the article goes on to look at some of the most important rules of IHL governing the conduct of hostilities and the interpretation in the cyber realm of those rules, namely the principles of distinction, proportionality, and precaution. With respect to all of these rules, the cyber realm poses a number of questions that are still open. In particular, the interconnectedness of cyber space poses a challenge to the most fundamental premise of the rules on the conduct of hostilities, namely that civilian and military objects can and must be distinguished at all times. Thus, whether the traditional rules of IHL will provide sufficient protection to civilians from the effects of cyber warfare remains to be seen. Their interpretation will certainly need to take the specificities of cyber space into account. In the absence of better knowledge of the potential effects of cyber warfare, it cannot be excluded that more stringent rules might be necessary.Keywords: cyber security, cyber warfare, cyber attack, international humanitarian law, cyber operations, cyber weapons, armed conflict in cyber space, conduct of hostilities, distinction, proportionality, indiscriminate attacks, precautions.