This open access volume is the first to provide a comprehensive overview of the ecological and environmental characteristics of marine and freshwater parasites. In three clearly organized sections, world-leading authors present the current state of our knowledge as well as the future trends for their respective fields in aquatic parasitology. First, the basic life cycle strategies of the various major groups of aquatic parasites are presented, including protists, myxozoa, aquatic fungi, helminths, and arthropods. Subsequent chapters explore the ecological implications of aquatic parasites covering topics such as biodiversity, evolution, community structures, behavior, and conservation - to name a few. In addition, important developments in research are presented, such as the use of different molecular tools and eDNA in aquatic parasitology. The final section is devoted to the new field of environmental parasitology, where readers will find contributions on biological markers, invasive species, and bioindicators of environmental pollution, among others. Due attention is also given to zoonoses, aquaculture, and the effects of climate change on aquatic parasites. All chapters include original high-quality illustrations and never-before-seen photographs which complement the diverse aspects of aquatic parasitology described. Thus, this book is a must for every parasitologist and ecologist.