The Physiology of Cladocera presents foundational information on these increasingly important model organisms. This valuable summary is based on the world's literature, including Russian research not widely available until now. It offers systematically arranged data on the physiology of Cladocera, as well as discussion on research directions. Expert contributions in genetics, immunology, and cytology provide comprehensive insight into the state of knowledge of Cladocera and its underlying mechanisms. Cladocera crustaceans comprise a significant part of the natural communities and biological productivity of fresh waters. In recent decades, they have been the subject of global studies in systematics, genetics, and molecular, ecological, and evolutionary biology. They are used as "sentinel" organisms to assess water quality and the environment. In addition, the Daphnia genome (a genus within Cladocera) was recently sequenced and published. This has led to a rapidly growing awareness of the importance of understanding physiological processes as they relate to evolutionary and ecological genomics and ecogenomic toxicology. Despite the increasing use of Cladocera in research and study, physiological background information on these creatures is fragmentary. Hundreds of publications have been accumulated on their physiology, and a synthesis and general representation of the literature has been needed. The Physiology of Cladocera stands alone as a valuable and comprehensive offering in this area.