This book provides a comprehensive exploration of aquatic parasitology, addressing the ecological and environmental roles of marine and freshwater parasites. Spanning 23 chapters, it offers a detailed overview of parasites’ life cycle strategies, their ecological implications and their impact on human and animal health. The book fills an obvious gap in the literature by focusing on the environmental aspects of aquatic parasitology, integrating traditional ecological concepts with modern molecular techniques such as environmental DNA (eDNA) and stable isotope methodologies. It emphasises the importance of understanding parasite–host dynamics in the context of global change, including climate change and habitat destruction, and advocates for the conservation of parasitic species as vital components of biodiversity. By presenting the latest research developments and future trends, this volume aims to inspire further studies in aquatic parasitology, contributing to the sustainability and health of aquatic ecosystems worldwide. The book is a valuable resource for those seeking to understand the complex interactions between aquatic parasites and their environments and the broader implications for ecology, conservation and the one health approach.