Are British animals and plants being threatened by the invasion of alien species? This was the question addressed at a Scientific Meeting of the Zoological Society of London on 10 March 1998 in three talks exploring the potential impacts of invading Chinese mitten crabs in the Thames catchment (Paul Clark and Philip Rainbow, The Natural History Museum), introduced foreign crayfish (David Holdich, University of Nottingham) and alien terrestrial flatworms (Hugh Jones, University of Manchester).