The impact of the parasitic swimbladder nematode, Anguillicola crassus Kuwahara, Niimi & Itagaki, on European eel populations is assessed with reference to published research on its origin and rapid dissemination, life cycle and transmission dynamics, and its pathogenic effect. The parasite was originally endemic to East Asia, but has transferred from its native host, the Japanese eel, Anguilla japonica Temminck & Schlegel, to the European eel, Anguilla anguilla (L.) and American eel, Anguilla rostrata (Le Seur). Anguillicola crassus is a very successful colonizer and is now known to occur in four continents (Asia, Europe, Africa and America). The nematode can severely impair swimbladder function and has caused mortalities in both farmed and wild populations in the presence of other stressors. Anguillicola crassus may impair the capacity of European eels to complete the spawning migration, although direct evidence is not available to support this hypothesis. Areas for future research are recommended.