International humanitarian law can be generally defined as the body of international law governing the conduct of armed conflicts. The notion of armed conflict is therefore central to understanding this area of law. This is true both in terms of the philosophical underpinnings of international humanitarian law and in terms of the legal rules that set out the limits of its operation. The guiding principles of international humanitarian law arise from the need to place limits on the conduct of armed conflicts. These restraints are needed due to what we will call the morally exceptional nature of warfare. The philosophical concept of warfare will therefore be our first focus in this chapter. We will then turn to the legal limits on the operation of international humanitarian law. The fundamental principle here is that international humanitarian law has no operation unless an armed conflict exists. This raises the important question of what constitutes an ‘armed conflict’. We will therefore examine the legal definition of armed conflict, before turning to related questions concerning the duration and scope of hostilities. Finally, we will consider the legal distinction between international and non-international armed conflicts. This distinction has historically played an important role in international law. However, as we will see, its importance is diminishing over time.