There are 28+ native species of terrestrial flatworm in Europe with more being identified every year due to renewed scientific interest and developments in taxonomy. However, these native flatworm species are mostly small and innocuous animals. There are 21+ alien terrestrial flatworm species in Europe (includes four species only found in the UK) and some of these are much larger and damaging predators. For example, the invasive alien New Zealand flatworm (Arthurdendyus triangulatus) can reduce earthworm biomass by 20% in pasture leading to a reduction in grass yield of 7%. Arthurdendyus triangulatus is currently restricted to the British Isles and Faroe Islands but another large flatworm species, Obama nungara, is spreading in continental Europe and could have similar impacts to A. triangulatus. The key pathway for introduction of alien flatworms is plants for planting (nursery stock), especially containerised plants with growing media. The flatworms opportunistically shelter in plant containers and are not associated with a particular plant product. Very few control measures against invasive alien flatworms have been tested. Potentially, the most promising approach is hot water treatment applied as a drench. Flatworms are cryptic, nocturnal species and difficult for plant inspectors to detect. However, they would potentially be an ideal target for the development of environmental DNA detection techniques. Invasive alien flatworms are also spreading elsewhere in the world but no countries have specific phytosanitary treatments targeted at such species.