The diversity of ichthyofauna in Hellenic freshwaters is highlighted by the presence of 161 species. This actually represents 27.8% of the total fish species recorded in European freshwater, including both native and introduced species, suggesting that Greece represents a ‘hot spot’ for biodiversity. The introduction of alien species for restocking, the aquarium trade and aquaculture was responsible for the deliberate or accidental release of alien specimens into open waters and in some cases for establishing populations. In contrast, commercially valuable alien species (e.g. salmonids, cyprinids and more recently sturgeons) generated significant income to individuals, companies and to the national economy. This paper aims to review the situation regarding the presence of alien fish and crayfish species in Hellenic freshwaters and in particular in aquaculture and to flesh out issues concerning their aquaculture potential.