The thesis covers the subject of succession between international organizations from both the theoretical and practical perspectives. The study establishes first a conceptual framework by examining the notion of succession as it has evolved in municipal law and the law of state succession and explains how the term applies to succession between international organizations. Then, through numerous examples taken from international practice and court decisions, the study presents a typology of the forms of succession between international organizations, the legal basis of such succession and, finally, the effects of succession on functions, treaties, property, funds, liabilities and employees