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.