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A good understanding of the mechanisms and magnitude of impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for efficient prioritization of actions for prevention of new invasions or for mitigation measures. In this review, we have identified marine alien species of high-impact on ecosystem services and biodiversity in the European seas, classified the mechanisms of impact, commented on the methods applied for assessing impact and the related inferential strength, and reported gaps in available information. Furthermore, we have proposed an updated inventory of 86 marine species in Europe, representing 12 phyla, with documented high impact on ecosystem services or biodiversity. \nFood provision was the ecosystem service that was impacted by the highest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of species were ocean nourishment, recreation and tourism, and water purification, while the ecosystem services that were more often positively impacted were water purification, cognitive benefits, and climate regulation. In many cases, marine aliens impact keystone/protected species and habitats. Almost 30% of the assessed species affected entire ecosystem processes or wider ecosystem function, more often in a negative way. Fifty of the assessed species have been reported to be ecosystem engineers, fundamentally modifying, creating or defining habitats by altering their physical or chemical properties.\nPositive impacts of alien species are probably underestimated, as there is a persistent perception bias against alien species. We conclude that the “native good, alien bad” philosophy is a misconception and the role of most of the alien species in marine ecosystems is rather complex. Among the species herein assessed as high-impact species, 15 had only negative and 8 only positive impacts; for the majority (64 species) both negative and positive impacts have been reported, and the overall balance is often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only ~10% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform managers.