The important volume summarizes the current trends and developments in the study of bacterial and viral fish diseases. Books on these subjects are few and relevant review articles are mostly outdated. This volume will thus serve as a platform for scientists and aquaculturists to understand the current limitations as well as new developments so that fish health and disease control can advance to new heights.The first section provides readers with an overview of the bacterial and viral diseases and the current understanding of innate immunity and interactions with pathogens. Section II includes case studies, where three pathogens are presented, namely two bacteria (Aeromonas hydrophila and Vibrio anguillarum, the common causes of bacterial diseases in freshwater and marine aquaculture, respectively) and the white spot syndrome virus (an important viral disease in shrimp). These case studies serve as models for the study of various bacterial and viral diseases. Section III presents new platform technologies that are widely used in the study of human pathogens. It aims to spur fish biologists to use modern and cutting edge technologies for their studies so that the study of fish disease can move into the mainstream and focus on the basics. The final section is on marine biotechnology, discussing biotechnology products that are urgently needed for the aquaculture industry — spin-offs from basic research, including diagnostics, immunotherapy and vaccine development, and the use of probiotics.