Encyclopedia of Fish Physiology, Second Edition is a comprehensive, introductory-level reference work that aims to unite readers in the wonders of this discipline. 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. Final sections 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 greatly expands 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.