International humanitarian law (IHL) can be defined as the body of international law governing the conduct of armed conflict. It protects those not, or no longer, taking part in the hostilities and limits the means and methods of warfare. The four Geneva Conventions of 1949 (GCs) and their Additional Protocols of 1977 and 2005 (APs) are the central international treaties regulating IHL. These documents, in conjunction with customary IHL, establish a rule, applicable in both international and non-international armed conflicts, whereby States must not only respect IHL, but also ensure respect for IHL. This requirement is stated explicitly in Common Article 1 (CA1) of the GCs. The contributors to this volume consider the ways in which the requirement to ensure respect for IHL confers States with legal obligations in specific domains of conduct. This chapter sets the scene for those discussions by considering the historical development and key principles of IHL, focusing particularly on the GCs and APs. The chapter thereby aims to provide the necessary framework for understanding the significance of the other contributions in this volume.