Fish is the principal source of protein for people in many parts of the world, particularly in developing countries. While most fish are caught from nature, aquaculture or fish farming is now making a significant contribution to total fish production. More intensive conditions of aquaculture often result in a higher incidence of fish diseases and disorders. As in the first edition, the focus of this second edition is on protozoan and metazoan parasites that cause disease in fish. Significant changes to this second edition include the addition of three new chapters and four of the original chapters have new authors.