This chapter examines international humanitarian law (IHL), the body of international law that contributes to our understanding of the regulation of violence in international relations. It commences with an examination of the roots of international humanitarian law - that is, the institutional history of IHL and the sources of IHL. It then explores the purpose of IHL which is to regulate the use of deadly force by each and every party to an armed conflict. The chapter particularly focuses on the basic principles of IHL, which aim to strike a balance between military necessity and the principle of humanity during times of armed conflict. It concludes by discussing what reasons states have to comply with IHL and indicators of compliance.