The aim of the thesis is to clarify the status in international law of countermeasures by non-injured States in the enforcement of norms protecting fundamental community values. For this purpose, the thesis examines two sub-questions: First, whether and to what extent countermeasures by non-injured States as a response to violations of obligations erga omnes are tolerated in contemporary international law. By employing the concept of toleration introduced by Judge Simma in his separate declaration in the ICJ s 2010 Advisory Opinion, the question is not whether countermeasures in the general interest are legal, but rather whether they are not illegal . Understood as a property of fundamental norms, instantiations of obligations erga omnes are more easily identified through the better-developed concept of jus cogens. The prohibitions on genocide, war crimes and crimes against humanity are all peremptory norms owed erga omnes. In addition, the prohibition on genocide entails a positive extra-territorial duty to prevent genocide and there are indications that the same applies to serious war crimes and crimes against humanity. Despite the International Law Commission s compromise solution in its 2001 Draft Articles on the Responsibility of States for Internationally Wrongful Acts, existing State practice and opinio juris reveal a strong argument for the toleration of countermeasures by non-injured States in response to serious breaches of peremptory norms. Further, the legal regime regulating the measures and State practice do not lend strong support to the arguments against countermeasures in the general interest based on the risks of abuse inherent in self-assertion. Lastly, although international law contains a framework for extending countermeasures beyond principally responsible States, this extension reveals some legitimacy concerns for countermeasures by non-injured States in the general interest. The second sub-question of the thesis deals with the relationship between individual measures and collective enforcement of obligations erga omnes. It examines enforcement through the United Nations (UN) in light of the Responsibility to Protect policy developed since the 2005 World Summit. In this respect, the public law ideal of collective enforcement of community norms leaves a residual role for individual measures. Furthermore, in cases where there is need to coordinate responses or secure legitimacy, the UN General Assembly is competent to assume a more prominent role in recommending individual countermeasures by non-injured States.