About 10,000 alien species have been registered in Europe. Some of these species were introduced on purpose and remain economically important. However, a proportion of the alien species established cause significant damage to native biodiversity and can be classified as invasive alien species according to the Convention on Biological Diversity.

Invasive alien species (IAS) are non-native species whose introduction and/or spread outside their natural, past or present, ranges pose a threat to biodiversity.

Invasive alien species occur in all major groups, including animals, plants, fungi and micro-organisms, and are considered, at least on islands, to be the second most important reason for biodiversity loss worldwide (after direct habitat loss or destruction).

Invasive species can cause great damage to native species by competing with them for food, eating them, spreading diseases, causing genetic changes through inter-breeding with them and disrupting various aspects of the food web and the physical environment.

There is an upward trend in the establishment of new species, with impacts on biodiversity expected to increase because of the growing number of species involved, and an increasing vulnerability of ecosystems to invasions, which results from other pressures such as habitat loss, degradation, fragmentation, over-exploitation and climate change.

Globalization, particularly increased trade and tourism, have resulted in an upsurge in the number and type of alien species arriving in Europe. Marine and coastal areas are being drastically affected as a result of increased shipping and the building of canals between isolated seas. In order to gain better understanding of invasive alien species and their impact on European biodiversity a list of the worst invasive alien species threatening biodiversity in Europe has been established. The list currently contains 163 species or species groups.