Fish diseases play a major role in aquatic ecosystems. Both wild and cultured fish suffer from a number of parasitic, bacterial, fungal, and viral diseases. Their impact can most clearly be seen among aquacultured fish. However, wild fish also suffer from a range of diseases that affect their survival. Infections may even influence quality parameters (texture, edibility) and thereby affect industrial exploitation. The present book outlines important aspects of the diagnosis, life cycles, symptoms, prophylaxis, and control of fish pathogens. Because many fish pathogens are species specific, the authors do not attempt to catalog all fish diseases, rather, the text should be regarded as an introduction to the main areas by providing a series of relevant examples of host-pathogen systems. This text will be useful for a range of professionals and students working with the aquatic environment. Readers interested in aquaculture, the biology of fish, fisheries biology, and education may find it useful as an introduction to the field. The authors are educators, researchers, and trainers who have been working with fish disease at the University of Copenhagen for over twenty-five years.