Most scientists agree that the introduction of exotic species in aquatic environments is a risky business that can cause several problems. Diseases and parasites may be introduced into the new environment, the introduced species may be accompanied by less desirable species, the introduced species may out compete the indigenous fauna, the food web may change to a less favourable status, the introduced species is usually impossible to eradicate. The long-term gain, if any, is very difficult to foresee and it is said that the real judgement of an introduction cannot be made until 100 years have passed. However, there are good examples of species which seem to have acclimatized to the new environment without any ecological disadvantages, e.g. rainbow trout and some carp species. Successful introductions of freshwater crayfish have also been made in the past. Pacifastacus leniusculus was introduced during the period 1895–1916 into Lake Tahoe in California. Specimens from this population were brought into Sweden in Europe in the 1960s and later from Sweden into several other European countries. In some countries, the species has established viable populations. In the 1970s, Procambarus clarkii was introduced into Spain from Louisiana, USA. This latter species now dominates the harvest of crayfish in Europe. The objective of this paper is to evaluate the commercial, socio-economic, recreational and biological values of the introduced crayfish species in Europe. The positive impact of introduction of alien crayfish in Europe can be summarised as: 1) Rapid restoration of traditional habitats, e.g. fishing for crayfish in Sweden; 2) Economic benefit for local crayfishermen, e.g. Spanish netsmen; 3) Diversification of agriculture to include astaciculture, e.g. crayfish farmers in Britain and in Sweden; 4) Developmental research into astaciculture, e.g. various universities and institutes including commercial companies, e.g. in Sweden; 5) Creating large artificial waterbodies; 6) Restocking of lakes with plague-resistant species, e.g. Finland and Sweden; 7) Preventing lakes from being overgrown by water plants, e.g. Finland and Sweden; 8) Socio-economic advantages for rural people; and 9) Increased trade between countries inside Europe as well as between European countries and countries outside Europe.