The Hydrobiidae is the most diverse family in the West Palaearctis with 148 genera and more than 740 species which have been depicted here with nearly 2050 photos, many of them displayed for the first time. The problem was to find and end writing this book because because many species have been described as new during writing this book. So new species have been considered until the end of February 2021, knowing that there are some more new species under description. As done in the 2nd volume it has been predominantly worked with syn-, holo-, para- and topotypes if possible. Identifications with type materials are important, because many species look similar at a first glance but are distinct in fact. Treated genera: Heideella, Rifia, Bullaregia, Aghbalia, Atebbania, Navalis, Kaskakia, Syrofontana, Pseudamnicola, Corrosella, Didacus, Falniowskia, Pontohoratia, Motsametia, Sitnikovia, Schapsugia, Tachira Kartvelobia, Imeretiopsis, Caucasopsis, Caucasogeyeria, Hausdorfenia, Karucia, Gloeria, Dalmatinella, Prespiana, Narentiana, Zavalia, Pseudavenionia Myrtoessa, Sumia, Kirelia, Radomaniola, Grossuana, Antibaria, Belgrandia, Guadiella, Plesiella, Litthabitella, Terranigra, Vracenica, Malaprespia, Trichonia, Graecorientalia, Pseudorientalia, Pseudoislamia, Falsibelgrandiella, Daphniola, Prespolitorea, Ohridohoratia, Ohridohauffenia, Ohrigocea, Gocea, Dalmatella, Hauffenia, Agrafia, Hadziella, Tarraconia, Lyhnidia, Pseudohoratia, Strugia, Zaumia, Graecoanatolica, Anatolidamnicola, Sivasi, Turkorientalia, Belgrandiella, Tschernomorica, Balkanica, Kolevia, Graziana, Pontobelgrandiella, Cavernisa, Devetakiola, Stoyanovia, Sarajana, Anangelovia, Istriana, Arganiella, Docleiana, Aretiana, Heraultiella, Plagigeyeria, Travunijana, Lanzaia, Lanzaiopsis, Saxurinator, Cilgia, Meyrargueria, Palacanthilhiopsis, Microstygia, Iverakia, Zeteana, Devetakia, Phreatica, Kerkia, Horatia, Sardohoratia, Pezzolia, Sadleriana, Sheitanok, Tanousia, Graecoarganiella, Anagastina, Vinodolia, Deganta, Islamia, Tefennia, Ifrania, Fessia, Idrisiella, Pikasia, Salaeniella, Milesiana, Ismerope, Josefus, Pauluccinella, Spathogyna, Iberhoratia, Avenionia, Corbellaria, Fissuria, Boleana, Alzoniella, Boetersiella, Chondrobasis, Pyrgohydrobia, Pyrgula, Dianella, Chilopyrgula, Stankovicia, Neofossarulus, Trachyochridia, Ginaia, Ohridopyrgula, Xestopyrgula, Micropyrgula, Falsipyrgula, Clathrocaspia, Dabriana, Nicolaia, Torosia, Shadinia, Sarkhia