Ostracoda are small, mostly 0.3-5 mm long crustaceans the body of which is totally enclosed within a carapace consisting of two calcified valves. Only the tips of various appendages protrude through the narrow gap when the valves open. Ostracods are found living in both marine and non-marine environments. They occur in almost every type of freshwater habitats, whether permanent or temporary, epigean or hypogean, stagnant or flowing. They colonize springs, ditches and canals, (rock)pools, slow rivers, man made basins, the aquatic interstitial habitats, cave waters, ponds and lakes where they are found from the shallow littoral down to the great depths of the profundal zone. A number of species are ecologically specialized and are considered indicators of various environmental conditions, mainly temperature, depth and salinity. The present work provides a practical guide to the freshwater and slightly brackish water species recorded in western and central Europe from the British Isles and the northern half of France to the Czech Republic and Slovakia. Keys to all taxa are given. The treatment of each of the 158 species recorded includes descriptions and illustrations of both the carapace and appendages as well as summaries of the ecology, life history, distribution and fossil record. It should be of value to both beginners and experienced workers, freshwater biologists and ecologists.