The invasion of exotic species has always been an intriguing and important ecological topic. With the advent of modern molecular techniques that enable the transfer of genes from one organism to another, whether or not they are closely related, we have a new class of ‘exotic’ organisms which may escape or be released into the environment. What will be the consequences of such new invasions? Will they be more or less damaging than the more traditional foreign exotics? The purpose of this fourth Ecological Issues Booklet is to address some of these questions.