Genetic transformation has allowed crucifer species improvement. Because of pollen and seed dispersal of these species, and of the presence of a large number of wild crucifers into the cultivated areas, transgene flow from crops to weedy relatives has to be assessed under natural conditions. It depends on the possibility of Fl interspecific hybrid production, on the fertility of successive backcross progenies, of gene exchange between the genomes and on the maintenance of introgressed weeds or of new weedy species. The ploidy level and genomic structure of the original species as well as the location of the transgene on the crop genome influence gene flow. The different situations between diploid species, diploid-tetraploid species and between tetraploid species will be presented and discussed.