Since the beginning of the 19th century, alien crayfish have been introduced into Wallonian Belgium, France, and Luxembourg. The main alien species belong to the American genera: Orconectes, Pacifastacus, and Procambarus. The alien species, Astacus leptodactylus, has also been introduced from Turkey. All are pioneer species and consequently severe competitors of the native species, Astacus torrentium and Austropotamobius pallipes. Some alien crayfish may be vectors of diseases, such as the crayfish plague fungus. Aphanomyces astaci. The genus Procambarus has a negative action on the environment due to its burrowing in dams and dykes. The national and European laws are not sufficient to regulate the populations of alien crayfish. An effort must be initiated by the scientific community to change a very bad ecological situation.