Various aspects of original ecosystem state and non-native species invasions in the Black Sea are discussed in details, including history of the Black Sea basin and its biota, main features of the Black Sea physical geography, hydrology and hydrochemistry, ecosystem state before the non-native species invasion, non-native species introduction in the Black Sea, and non-native biota composition. The Black Sea is considered as the main recipient region and, at the same time, a donor of non-native species. Special attention was paid to invasive ctenophores Mnemiopsis leidyi and Beroe ovata and the role of the latter as a bio-controller of M. leiduyi population. Also, the Marmara and Aegean Seas as the nearest neighbors of the Black Sea and the connection route between the Black and Mediterranean Seas were distinguished. Their water exchange with the Black Sea, biota composition, non-native species, and ecosystem state have been considered.