Fundamental breach, as a central concept of CISG, is discussed in detail, not only in Art.25, but also in other parts of the convention such as Art.71 and Art.72. The system of fundamental breach of contract is the cornerstone of the system of remedies for breach, and it holds a significant position in the field of international commercial transactions. What is anticipatory breach? What is anticipatory fundamental breach? How to define substantial detriment in the CISG? What is "substantial" deprivation to a "serious" extent? Whether the defaulted party substantially deprives the future expectation interests of the injured party to a "serious" extent is the key to distinguishing between anticipatory fundamental breach and anticipatory non‐fundamental breach. The substantiality and seriousness elements are the essential criteria in anticipatory fundamental breach of contract under CISG.