Alien species represent a growing threat to marine environments due to their potential to change the structure and species composition of the ecosystems they invade. The ctenophore Mnemiopsis leidyi, notorious for wreaking havoc in the Black Sea and ranke d among the world's most detrimental invasive species, was first observed in northern Europe in 2006. It has since been thriving, spreading and proliferating, and can probably now be considered established in the Baltic as well as the North Sea, including the Norwegian south coast. Abundant Mnemiopsis can severely disrupt pelagic food webs through direct predation and competition as well as several top-down and bottom-up processes impacting both the classic food chain and the microbial loop. Nevertheless, the occurrence and impact of Mnemiopsis in Norwegian waters is poorly known. Due to Mnemiopsis' proven ability to disturb pelagic ecosystems, it is imperative to study its role in this new habitat. The conditions Mnemiopsis faces in Norway differ signifi cantly from those in its native range along the east coast of the Americas, in the Ponto-Caspian region or in the Baltic. Experiences gained from these regions are thus not directly transferable. Our aim is to investigate the geographic and seasonal distr ibution of Mnemiopsis in Norway and to evaluate its potential ecosystem impact. We will accomplish this through a combination of monitoring activities, field studies and experiments targeting local reproductive and feeding biology of Mnemiopsis, predation on Mnemiopsis, and the cascading impacts of abundant juvenile and mature Mnemiopsis on different levels of the pelagic community. The resulting knowledge will improve our ability to understand and predict changes and succession in pelagic ecosystems due to invasive species and/or increased numbers of gelatinous predators, in Norway as well as globally. The results are relevant with respect to prudent ecosystem-based management of Norwegian marine resources.