This paper reviews the alien polychaete species in the world's oceans and their impacts on the marine ecosystems and humans. A total of 292 polychaete species belonging to 164 genera and 39 families have been transported around the world's oceans with human-mediated assistance. Almost 15% of total number of known polychaete genera and 3.4% of total number of species are included in this phenomenon. A total of 180 species have become established in the world's oceans and 31 species (casual species) have a potential to establish viable populations in a new location. The most speciose genera are Hydroides (16 species) and Polydora (16 species), both accounting for 10% of the total number of alien species. The families Spionidae (53 species) and Serpulidae (46 species) have the highest number of alien species. The Mediterranean Sea (134 species), and the coasts of the Hawaii Islands (47 species) and the USA Pacific (34 species) have been intensively invaded by alien polychaetes. The origins of alien species vary among regions. Alien polychaete species in the Mediterranean Sea mostly originated from the Red Sea and Indo-Pacific areas. Benthic habitats of the areas between 40 degrees N and 40 degrees S were colonized by polychaetes mostly originating from other tropical and subtropical regions. The Suez Canal and shipping are the major vectors for species introductions. Some species imported and exported as fishing baits have become established at non-native localities. The invasive polychaete species have greatly altered habitat structures in some areas, restructured the food webs, and created important economic problems.