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, etc.) and thereby affect industrial exploitation. This book outlines important aspects of the diagnosis, life cycles, symptoms, prophylaxis, and control of fish pathogens. The text provides a series of relevant examples of host-pathogen systems. The contributors are educators, researchers, and trainers who have been working with fish disease for over 25 years. The book will be useful for a range of professionals and students working with the aquatic environment.