Textbooks on international law, dicta of the ICJ and the International Law Commission's 'Guiding Principles applicable to unilateral declarations of states' of 2006, all reflect the fact that in international law a state's unilateral declaration can create a legally binding obligation. Such declarations are common, as a look at the weekly headlines of any major newspaper will reveal. Many of the declarations made at the highest level are, of course, vaguely expressed and carry no tangible legal commitment. But others deliver a very clear message: for instance the US's April 2010 declaration on its future use of nuclear weapons or Kosovo's declaration of independence and pledge to follow the Ahtisaari Plan, are two recent and prominent examples of unilateral declarations at the international level. The sources, however, also reveal that while state promises are accepted as a means for states to create full blown legal commitments, the law governing such declarations is far from clear. This monograph fills a gap in international legal scholarship by raising and answering the question of the precise legal value of such pledges in the realm of public international law. After a brief introduction state promises in international law are defined and contrasted with other unilateral acts of states, and the history of promises in state practice and court decisions is delineated, together with scholarly opinion. The book then provides a detailed picture of the international legal framework governing promises of states, and ends with a brief assessment of the raison d'etre for promises as a binding mechanism in international law, and their disadvantages in comparison with the classical mechanism for assuming international obligations - the international treaty. This the first book to present a comprehensive overview of the legal effect of promises by states in international law.