Focusing on pathobiology and protective strategies against protozoan and metazoan parasites of fish, this book reviews the latest research on important parasites: those that cause financial hardships to the aquaculture industry, have been introduced to new geographical regions through transportation of infected fish, are pathogenic to groups of finfish and detrimental to production, are highly adaptable and not host-specific with worldwide distributions, and that may serve as disease models for studies on other pathogens. It also highlights gaps in the knowledge to help direct future research.