This textbook provides a highly accessible and concise overview on the innate and adaptive immune systems in fish as well as on fundamentals and latest developments in fish vaccinology. It introduces the anatomy and molecular functions of immune organs and furthermore examines in detail the interactions between the host immune systems and different types of pathogens.The textbook is essential reading for students in Veterinary/Fish Medicine, Aquaculture and Immunology. Furthermore, the volume serves as a quick reference for Fish Pathologists and Aquaculturists.Chapter 2 is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.