Pathways and vectors of non-native species introduction in the Ponto-Caspian seas are reviewed. Appearance of new species in the aquatic ecosystems may be the result of a spontaneous penetration via the sea straits and rivers (natural range expansion, which is a frequent phenomenon) or, most commonly, as a result of human activity. The following root causes (vectors) for the appearance of non-native species are considered: natural expansion via existing waterways; aquariumistics; aquaculture; intentional introduction of commercially valuable species; accompanied releases; construction of canals and reservoirs, that opens up new highways to the penetration of non-native species into new basins, fully related to unintentional human intervention; shipping, including transportation of invasive species with the ballast waters or as fouling communities. Consequences of the most prominent canals construction (i.e., Volga–Don Canal and Suez Canal) are discussed. Main invasive corridors for the species invasion in Europe (Southern, Central, and Northern) are distinguished. It is pointed out that the species invasion processes accelerated dramatically in the past few decades due to new technologies, shipping intensity increase, global expanding of shipping routes, increased basin connectivity, and global warming.