This chapter defines Official Development Assistance (ODA) and situates it within the wider universe of international cooperation. It reviews the main concepts, classifications and modalities of ODA as established by the OECD’s Development Assistance Committee and by national cooperation agencies. Particular attention is paid to the distinction between bilateral and multilateral aid, as well as to emerging forms of North–South, South–South, triangular and decentralized cooperation. The chapter then examines the specific institutional architecture of Japanese ODA, describing its objectives, instruments and decision‑making processes, as well as the key actors involved in policy design and implementation. It highlights the centrality of economic infrastructure, technical cooperation and debt relief in Japan’s development cooperation and shows how these tools have been deployed in different regions. By establishing a clear conceptual and institutional framework, the chapter provides the analytical foundations needed to interpret Japan’s ODA flows and their link with economic diplomacy in later chapters.