Fish farming, in seawater and in freshwater, in cages, tanks or ponds, makes an ever-increasing and significant contribution to the production of aquatic food in many regions of the world. During the last few decades there has been significant progress and expansion in the aquaculture sector, characterized by intensified production and the exploitation of many new species. Aquaculture must be a sustainable bio-production, environmentally as well as economically. Disease prevention in order to reduce losses, and the use of antimicrobials is crucial in this perspective. Vaccination has, in a few years, become the most important method for disease prevention in aquaculture, and effective prophylaxis based on stimulation of the immune system of the fish is essential for further development of the industry.