As evidenced by the December 2004 tsunami in the Indian Ocean, the forwarding and dissemination of information is an essential element of natural disaster response. One component of an effective natural disaster information system is the flow of data between states on an international level. Though, in international environmental law, it has been discussed at length whether a duty to inform exists when a technological accident threatens to harm the environment of neighbouring states, little has been said on the question if states are obliged to warn each other if they possess information about an impending natural disaster. Such an obligation is embodied in Rio Principle 18; since the Rio Declaration has no binding force, however, the question still begs an answer.There already exists, especially in Europe, a sizeable number of bilateral agreements on humanitarian assistance in cases of disasters and emergencies which also provide for the forwarding of information on natural disasters; multilateral agreements figure less. But even in the absence of treaty obligations, states are under a universal customary obligation to warn each other of impending natural disasters if they possess such information. State practice is based on a corresponding international opinio iuris. Indications as to this can be found in international soft law, especially the Declarations of Rio (1992), Yokohama (1994), and Kobe (2005), as well as a number of resolutions and drafts by U.N. organs and their subsidiary bodies. These indications are further supported by the fact that such a duty to inform follows from a universal positive obligation of states to protect human life. In cases of emergency and disaster this obligation belongs to the minimum core content of social and civil rights alike, as far as no or negligible costs are involved on the side of the state. This minimum obligation was hinted at by the International Court of Justice in the Corfu Channel judgment as an expression of »elementary considerations of humanity«. Regarding developing states, the duty to inform is also supported by their preferential legal position in the area of non-commercial transfer of knowledge.According to the rationale of this duty and in accordance with the principle of good faith in the performance of international obligations states are obliged immediately to forward all relevant information to states threatened by natural disasters. Since in practice it will be difficult to impute certain damages to the state obliged to inform, the breach of this obligation will only give rise to a right to demand satisfaction. Nevertheless, as states are bound to inform immediately, this entails a duty to provide for competent authorities and to reduce delays within the own jurisdiction. By means of these preliminary duties, already the existence of an obligation to inform can prompt states to open up channels of information, thus contributing to an effective natural disaster information network.