Alien species are the species who appear beyond the distribution range that have been historically reported. They may belong to different taxonomical groups, such as fish, crustaceans, algae, bacteria etc. The main proportion of alien species arriving in Greek waters originate from the Red Sea and are called Lessepsian migrants. This is in honor of Ferdinand de Lesseps, a French diplomat in charge of the Suez’s canal construction in 1869, through which the species manage to enter. Once they enter to Mediterranean many of them fail to survive in the new environment. However, the increase in the surface water temperature as a result of climate change, facilitates the acclimation and the successful spreading of some adaptable species to the new environment. In several cases they coexist harmoniously with native species. Although, some adapt more effectively to the new conditions than the native species, resulting in rapid spreading and increase in population numbers. The latter are called invasive species and have negative effects on a region’s economy, environment and/or public health. According to the Greek Institute of Marine Research and previous surveys performed in the Mediterranean Sea, they have been recorded more than 986 marine alien species, of which 237 occur in Greece. Nowadays, a number of International Conventions recognize the alterations that can occur to local ecosystems by the introduction of alien species, while the EU now includes the management of alien species in the strategic European policy objectives for aquatic ecosystems. In this context, iSea aims through its projects to improve the study of the distribution routes and the biology of alien species, and help in finding solutions to mitigate their impact on aquatic ecosystems in Greece.