Superimposed on the natural distribution of freshwater crayfish is one orchestrated by humans, mainly from deliberate introductions to satisfy demand for this food, although sometimes accidentally via fishing bait and other means. While some crayfish introductions had positive effects when they became established, in the majority of cases they also had negative effects, which often outweighed the positive ones. These negative effects include displacement of native crayfish species, transfer of disease, consumption of fish eggs, reduction of fish stocks, consumption of large amounts of invertebrates and macrophytes, upsetting production in rice fields, and displacement of amphibians. In addition, the burrowing activities of some species can cause physical damage to irrigation structures and banks of rivers and lakes. All these factors cause environmental degradation of freshwater habitats. Unfortunately, such changes cannot be easily rectified as without extreme measures it is almost impossible to remove established alien crayfish populations. The main species causing concern are Pacifastacus leniusculus in California and Europe, Orconectes limosus in Europe, Orconectes rusticus in North America, Procambarus clarkii in Africa, California, Europe and many other parts of the world, Astacus leptodactylus in Europe, and Cherax destructor in Africa and Australia. All these crayfish are invasive and build up their numbers very quickly. Not all problems relate to trans-continental introductions but inter-state introductions can be equally damaging when native crayfish are moved outside their home ranges as is happening within North America, Australia, and Europe. Legislation is in place in many countries to try and curtail the spread of alien crayfish, but this is often ineffective against those wishing to propagate a valuable crayfish species. Educating governments and the public about the negative impacts of crayfish introductions can play an important role in this respect. Degradation of the freshwater environments is set to continue unless means can be found to control, if not eradicate, nuisance populations of alien crayfish.