Hostile activities conducted by exiled dissidents from foreign sanctuaries frequently lead to conflict between the asylum State and the State of origin. This article considers the duty owed by an asylum State to the State of origin under international law. Two factors limit the asylum State's legal obligation. First, international law predicates State responsibility for the acts of private persons, such as foreign exiles, upon the existence of fault. The asylum State must either contribute to the forbidden conduct, or possess the knowledge, opportunity and capacity to prevent it and fail to do so. Geographic and resource limitations excuse liability, while political constraints do not. Secondly, the conduct must violate international law. Not every action considered hostile by a State of origin injures its sovereign interests. Exile political organizations, for example, are essentially a manifestation of the exercise of basic human rights. International law grants an asylum State substantial discretion to limit the political freedoms allowed to foreign nationals, 6ut permitting their exercise gives the State of origin no legitimate cause for complaint. The provision of financial and material assistance, hostile propaganda and other forms of non-violent support to subversive groups active within the State of origin has a more complex legal character. As purely humanitarian assistance is unlikely to alter the course of a civil conflict, an asylum State may permit or even encourage such efforts. Asylum States ought not to support more openly hostile acts of non-violent subversion, but international law at present imposes no active obligation to prevent similar conduct by private persons. Recent practice, however, reveals a possible trend towards a more comprehensive duty of prevention. The question of asylum State responsibility for the violent subversive acts of foreign exiles is unsettled in significant respects. States have vigorously condemned such attacks and, through United Nations General Assembly resolutions, have repeatedly recognized the existence of an active duty of prevention, except where self-determination is at issue. Many of these same States, however, at some point, sponsored or tolerated the activities of terrorists, armed bands or insurgents. The decision of the International Court of Justice in the Nicaragua case suggests a basis for arguing that, despite the evidence of contrary practice, States now recognized and accept that such support or acquiescence is illegal. The extent of disregard for the norm in practice, however, makes it difficult to assert that the duty has attained the status of customary international law