In 1984 aquaculture production data from the Mediterranean were reported to FAO on only six species, mostly shellfish and a few mullet; culture of seabream and seabass was just beginning. Today there are between 20 and 30 Mediterranean farmed species for which data are reported. The application of technologies applied to modern breeding can be divided into two broad groups: those for short-term and those for long-term improvement. Hybridization, chromosome set manipulation, and sex reversal can be considered short-term improvements made over 1 - 2 generations; the improvements generally are non-cumulative, i.e., one time events. Selective breeding represents a long-term improvement programme where small gains accumulate over generations; gene transfer may be considered long-term where the gains may be substantial, but may not accumulate each generation.. As breeding of aquatic species becomes easier and more aquatic species become domesticated, genetically differentiated strains will undoubtedly increase and aquaculture development will be faced with the problem of how best to manage and promote the new diversity, while conserving the natural genetic diversity of aquatic species. Socio-economic, as well as technical and biological factors will play a vital role in this regard.