Their story began 500 million years ago, but we only started to get to know them in 1746, when Linné described the first ostracod species. Vividly portraying the freshwater ostracods, this comprehensive reference work gathers the knowledge gained during some 250 years, but which to date has remained scattered throughout the literature. It starts with an introduction to the class Ostracoda with a special focus on freshwater ostracods and highlights practical methods in their study. The systematic part includes an introduction to all families; identification keys for all subfamilies, genera and species; diagnoses for each subfamily and genus; and lists of synonyms and distribution of species. The text is richly illustrated with distribution maps, line drawings of key generic characteristics, and numerous SEM photographs. Serving as an excellent starting point for all further research on freshwater ostracods, it can be widely used not only by ostracodologists, but also by crustaceologists, evolutionists and ecologists.