With a continued increase in the range of transgenes, and plant species for which genetic modification is possible, this review attempts to bring together some of the factors that will influence the eventual fate of transgenes in the environment, and the effects that such a dispersal may have. The review is developed from papers presented at the SEB Swansea meeting (April, 1994). Using experiments with GM (genetically modified) plants, and markers in non-GM plants, as well as observations on natural and crop populations, it is possible to predict isolation distances required for limiting the unintentional release from GM crops, and the probable fate of both GM pollen and seed if it is released beyond the GM plot. Knowledge of wild relatives of crop plants, and ecological mechanisms can also give insights into the possible effects of different transgenes on native plants, and other agricultural crops. A large number of limited scale releases of GM plants have now taken place from which we can gain information on the performance of GM crops in an agricultural environment, and the stability of the GM phenotype. All this information, can help to form a sound basis for regulations on the release of GM plants, an assessment of the need for, and scope of monitoring, and the best way in which to use GM crops.