Abstract In the last decade society has become aware of the increasing negative effects of human waste products introduced to the oceans. There is proof evidence, at least for some areas of the world ocean, that the marine environment is seriously in danger. The scientific community is very concerned, arguing that there is an urgent need for basic research in this field because too little is known on the harzardous effects of man-made pollutants on the structure and functioning of marine ecosystems. There are two ways to perform experiments under controlled environment conditions: (1) in the laboratory; (2) in in-situ experiments with enclosures. Most laboratory experiments are designed to study the influence and the tolerance spectrum of specific pollutants, e.g. copper or DDT, on any specific organism, e.g. a mussel or a fish. In these experiments it is fairly difficult to simulate natural conditions. The concentrations of the pollutants are generally fairly high, often several orders of magnitude higher than in the ocean. It is questionable, if the results from these experiments can be extrapolated to nature. The second approach to obtain reliable data on the influence of perturbations on marine organisms is to use enclosures of various sizes in-situ or in landbased facilities. Fibre-glass containers and plastic bags have been used successfully in the last years, e.g. in the U.K., U.S.A., Canada, France and W. Germany. The main goal of these experiments is to study the long-term effect of low-level perturbations on natural populations of the pelagic or benthic ecosystem. In the paper examples of recent results will be discussed in detail.