In order to assess the risks of hybridization between genetically modified crops and closely related weeds, recent studies of introgression of crops and wild relatives growing nearby, using molecular markers, are discussed. Where introgression has occurred, the products have tended to colonize agricultural or disturbed habitats. However, if genetic modification of crops alters the probability of hybrid formation or the performance of hybrids, then natural and seminatural habitats may be invaded by hybrids carrying the modification. It is hoped that data on the maintenance of disease resistance polymorphisms in natural populations will prove helpful in modelling how transgenes conferring disease resistance might behave in natural populations.