A public international organization is formed by international agreement made by states or other international organizations. This thesis examines the different types of international organizations and the requirements that must be met for the international organization to act under international law. When an international organization receives international rights and obligations, the international organization becomes an international person. This personality is analyzed and differentiated from that of a state. The granting of rights and obligations to an international organization also means that the organization will be liable for its actions and those of its organs for breaches international law. Member states recognizing an international organization may also be liable for violations by the organization in two limited circumstances: external ultra-vires acts of the organization and ultra-hazardous activities where international law imposes absolute liability on member states. Member states are jointly and severally liable in these situations.