This chapter examines the application of international humanitarian law to cyber warfare in the sense of rising to the level of an armed conflict. Building upon the work of the Tallinn Manual on the Application of International Law to Cyber Warfare, it places particular emphasis on the application of the principle of proportionality and the duty to take precautions in conducting an attack in relation to attacks carried out in the cyber domain. In doing so, a preliminary examination is made of the likelihood of cyber warfare actually taking place and the scenarios which seem most likely for this to occur. It then goes on to examine how the aforementioned principle of proportionality would apply in relation to attacks whereby physical effects resulted and to attacks which simply had the effect of destroying or degrading digital data. It concludes by finding that while many cyber-attacks would not qualify as attacks under IHL, some would and that the law is applicable by analogy in much the same way it applies to conventional attacks by kinetic weapons.