The concept of obligations erga omnes - obligations to the international community as a whole - has fascinated international lawyers for decades, yet its precise implications remain unclear. This book assesses how this concept affects the enforcement of international law. It shows that all States are entitled to invoke obligations erga omnes in proceedings before the International Court of Justice, and to take countermeasures in response to serious erga omnes breaches. In addition, it suggests ways of identifying obligations that qualify as erga omnes. In order to sustain these results, the book conducts a thorough examination of international practice and jurisprudence as well as the recent work of the UN International Law Commission in the field of State responsibility. By so doing, it demonstrates that the erga omnes concept is solidly grounded in modern international law, and clarifies one of the central aspects of the international regime of law enforcement.