This is the first identification key for the freshwater gastropods of the Western Palearctic, reviewing the progress of malacological research of the last few decades. Nearly all species have been depicted by high-quality photos in a way that allowed species identification. Additional descriptions and identification keys, which refer to the typical characters of the species, support the illustrations. For most species, photos of anatomical details are provided. Altogether there are 424 photo plates with more than 1,600 individual photos. The basis of the species considered in this book are the lists of the Fauna Europaea project, published by Ruud Bank (2004 and 2011) as well as Bank & Neubert (2018) with additions of species which have been described after 2011 and Vinarski & Kantor (2018) for European Russia. This checklist has been expanded by species living outside Europe. The borders of the Western Palearctic as defined here are Iceland in the north, the Ural, along the Caspian Sea to Iran in the east, the Near East and Northern Africa to Portugal in the west. In addition species names in that are doubtful have been compared with WORMS (World Register of Marine Species). Volume 1 covers the Neritidae, Hydrocenidae, Ampullariidae, Viviparidae, Thiaridae, Potamididae, Melanopsidae, Bithyniidae, Cochliopidae, Tateidae, Hydrobiidae, Lithoglyphidae, Bythinellidae, Emmericiidae, Truncatellidae, Assiminiidae, Valvatidae, Lymnaeidae, Physidae, Planorbidae, Acroloxidae, Ellobiidae, and Otinidae.