Problems which might be caused by the release of transgenic organisms are discussed from the viewpoint of genetics, population genetics and ecology. On this background, the differences between "classical" methods to recombine genetic information and the more recent ones of gene engineering are pointed out and the predictability of gene interactions is discussed. Moreover, the possibility of uncontrolled reproduction of released transgenic organisms and the distribution of manipulated genes are demonstrated on the basis of our knowledge on nontransgenic organisms. Finally, the problems of risk assessment are emphasized.