In an anarchical society of independent states, unilateral measures play an important role. Fortunately, this does not mean that most states act unilaterally most of the time; on the contrary, cooperation is the key word in most areas of international relations and international law. However, in the absence of a comprehensive judicial system or effective enforcement system, states have and will continue to take unilateral measures to enforce law and policies. Sometimes these measures only claim to legality is the application of the countermeasures doctrine. This landmark book by Dr. Hjortur Bragi Sverrisson, the Head of Legal Affairs at EEA Grants, a development fund within the European Free Trade Association (EFTA), examines the law of countermeasures in the context of violations of international legal environmental obligations of states. It reviews the source of authority of states, namely, sovereignty, its interaction with the notion of international law, and the limitation of the latter. An in-depth review of the doctrine of countermeasures follows, by presenting, contrasting, and critically analyzing the views of the classical masters, as well as contemporary authors and other authoritative sources. The book puts the theories into context by presenting seven cases of states use, or threat of use, of unilateral remedies to protect environmental interests. One must remember that frustration, domestic politics, sovereign identity, and the need to show resolve not only are all a recipe for unlawful countermeasures, but also are often the underlying cause for such measures. Therefore, the environment for a solid and politically detached legal analysis regarding the flora of permissible measures might not be the most favorable. However, the decision to take countermeasures cannot be a spontaneous one; it has to be meticulously analyzed, and proper steps have to be taken before such measures are used. Only then can the countermeasures doctrine form the basis for a legal and legitimate unilateral enforcement of international law. By shedding a light on this labyrinth, this book provides guidance for scholars and students, private actors, and policy makers, as well as legal advisers to governments.