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 demonstrates 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.