Complicity in International Law aims to analyze questions arising from a state’s complicity in conflict with another state or an international organization. On the basis of international legal provisions, a state that assists the illicit fact of another state or an international organization in turn commits an offense if it is aware of the main fact and is bound by the same obligation. International law offers adumbrates the outcome of a codification process undertaken by the International Law Commission. The practice and its consequences, and the reflections of the doctrine, have matured with regard to the original hypothesis. Several cases of participation in the unlawful conduct of others, for example in facilitating the illicit use of the armed force, or of financial support to states responsible for human rights violations, have been recorded since the period immediately following World War II. International doctrine has long shown great interest in the theme of competition of several subjects in an international illicit act. This is a new phenomenon, given that until recently the issue had been the subject of in-depth analysis in a small number of works, few of which have been monographic in nature. Complicity in International Law will address the issue comprehensively.