With the rapid advancement of transgenic biotechnology, large numbers of transgenic crops have been produced and released for commercial cultivation (James, 2001), raising considerable biosafety concerns all over the world. One of the major issues is the potential ecological risk resulting from transgenes escaping into and persist- ing in the environment. The extensive on-going debate on this issue (Arriola, 1997; Wolfenbarger and Phifer, 2000; Crawley et al., 2001; Prakash, 2001; Dale et al., 2002) poses challenging questions regarding the research direc- tions that need to be taken to ensure that biotechnology outputs are responsibly deployed worldwide. Unless these biosafety issues are satisfactorily addressed, large-scale commercial release of the transgenic crops developed and further advancement of transgenic bio- technology are likely to be hampered. For an in-depth review of crop-to-wild gene flow, see Jenczewski et al. (2002) in this number of EBR. Here the more specific issue of the effectiveness of preventing gene flow from occurring will be discussed.