Seabass and gilthead seabream are the two marine fish species that have characterized the development of marine aquaculture in the Mediterranean basin in the last two decades. The substantial increase in production levels of these two high-value species was made possible by the progressive improvement in technologies for fry production in hatcheries. As a result, more than 100 hatcheries have been built in the Mediterranean basin, working on these and other similar species. At present, the farmed production of these two species derived from hatchery-produced fry is far greater than the supply coming from the wild. The first volume of this manual deals with the historical background and main factors influencing fish seed production; the life history and biology of the species; and hatchery production procedures. The second volume (ISBN 92-5-105304-9) deals with engineering and financial aspects of hatcheries.