The Encyclopedia of Fish Physiology is a comprehensive, introductory-level reference work that aims to unite readers in the wonders of this discipline. Fish live in almost every water system on this planet and represent more than half of all vertebrate species alive today. Fish physiology is a massive field of study and can be difficult to penetrate as most texts are written for readers with a post-secondary education in biology. This Encyclopedia aims to capture the interest of a more diverse group of readers, leading readers from core concepts to application in topics as far ranging as reproductive behaviours to surviving sub-zero temperatures. For example, from a foundation of heart anatomy, oxygen transport, and exercise physiology, readers can appreciate the challenges faced by migratory fish as river temperatures increase with climate change. The work aims and showing why biologists around the world choose to study fish physiology and what can be gained from such study. Chapters highlight where physiological systems and processes are conserved across vertebrate groups, which supports the use of fish as a vertebrate model for investigating fundamental problems in physiology and knowledge transfer to the medical field. Other chapters highlight unique specializations and adaptations that allow fish to survive in challenging environments, leading to a deeper appreciation of the natural world. Finally, other chapters demonstrate the consequences of perturbing physiological systems and how this knowledge can empower conservation strategies to protect fish. Much has changed in the field of fish physiology since publication of the previous, Prose award winning, edition. For example, we understand better the impacts of global climate change on the physiological systems of fish, and we have gained a deeper mechanistic understanding of physiological processes through technological advancements such as gene editing with CRISPR, whole genome sequencing, and quantitative ‘omic` approaches. The new edition will greatly expand the closing thematic section focused on applying fish physiology to real world challenges with topics including ocean acidification, declining habitat quality due to human activities, and the use of zebrafish in biomedical studies on tissue regeneration, neurological disorders and cancer. A new feature of the Encyclopedia is including presentation-ready summary slides of key concepts presented in each chapter, which will be a valuable tool for educators wishing to incorporate the Encyclopedia into class resources. All authors will be encouraged to enhance their chapters with multimedia features such as videos (ex. in fish behaviour chapters) and virtual microscope (ex. chapters on tissue morphology).