1. It has been suggested that genetic modification could lead to crops with enhanced invasiveness and persistence. These new genotypes could invade natural ecosystems and cause undesirable change, either through spread of the crops themselves or through hybridization with wild relatives. 2. We review the progress made in the genetic modification of the major UK crops, and identify those crops and traits in which genetic modification is most advanced. 3. Data on the potential for the spread of transgenes through pollen movement and the relative performance of modified and unmodified crops are examined. It is concluded that the spread of modified crops and their hybrids with wild relatives can be modelled in the same manner as for unmodified crops. 4. Evidence for hybridization between crop and wild species in the UK is reviewed. We identify three categories of crop according to the likelihood of formation of hybrids with wild relatives. 5. The categorization of crops is used to suggest some simplifications to the procedures for permission to release genetically modified crops into the environment. 6. We suggest research priorities for ecologists based on the traits that are of most interest to plant breeders using genetic modification and the most likely route of escape of transgenes from a particular crop.