This key provides a species-level overview of all (170) species of the Cladocera and Copepoda (Calanoida and Cyclopoida) known from Poland and surrounding countries (Middle and Eastern Europe). Keys in this book provide species descriptions limited to few key characters needed to the proper species identification. Keys to families, genera and species are arranged into plates with easy-to-use pictorial guide. This key should be relatively easy-to-use by almost any person, such as students and researchers with minimal or not at all expertise in identification of this groups. We were trying to use, whenever it was possible, almost a single character, in the easy pictorial way avoiding long verbal descriptions equipped with scientific jargon, to distinguish between similar species. The planktonic crustaceans belong to the most common freshwater invertebrates living in all kinds of aquatic ecosystems. They are important part of fish diet, can control algae blooms and are part of important mechanism of natural self-cleaning of aquatic environment. The book received a status of academic manual and the Polish Ministry of Science and Higher Education subsidized printing.