Recent years have seen an appreciable growth in the level of understanding of the dangers facing the international environment and an extensive range of environmental problems is now the subject of serious international concern. These include atmospheric pollution, marine pollution, global warming and ozone depletion, the dangers of nuclear and other extra-hazardous substances and threatened wildlife species. Such problems have an international dimension in two obvious respects. First, pollution generated from within a particular state often has a serious impact upon other countries. The prime example would be acid rain, whereby chemicals emitted from factories rise in the atmosphere and react with water and sunlight to form acids. These are carried in the wind and fall eventually to earth in the rain, often thousands of miles away from the initial polluting event. Secondly, it is now apparent that environmental problems cannot be resolved by states acting individually. Accordingly, co-operation between the polluting and the polluted state is necessitated. However, the issue becomes more complicated in those cases where it is quite impossible to determine from which country a particular form of environmental pollution has emanated. This would be the case, for example, with ozone depletion. In other words, the international nature of pollution, both with regard to its creation and the damage caused, is now accepted as requiring an international response.