The book tells about the birth, development and significance of hydronyms - the names of all water bodies of the Earth: rivers, lakes, seas, etc. Issues of hydronymics have not yet been specifically covered in popular science literature. Meanwhile, the study of them is important for a wide range of specialists both in the natural sciences and in the humanities. The practical value of hydronymics is for geologists and geographers who apply its data in paleogeographic reconstructions, geomorphological studies, exploration of mineral deposits, as well as for historians, linguists, ethnographers.