Chapter 2 defines humanitarian logistics. Section 1 underlines the crucial role of logistics and supply chain management in the humanitarian context; it identifies the main categories of disasters and describes disaster-relief operations. Section 2 delineates the phases that constitute the disaster management cycle; in particular, it identifies the specific phase of the humanitarian logistics stream that demand agile and lean principles. Section 3 indicates the different key actors in the humanitarian system, and it describes their role in disaster relief, underlining the complexity of humanitarian supply chain relationships. The chapter provides the reader with a brief introduction on the key concepts of humanitarian logistics and supply chain management, and underlines the complexity of an emergency relief operation.