The volume is the first in a series of publications under the common title Keys to Fish and Invertebrates of the Caspian Sea. It contains identification keys, illustrated in detail, for all genera and species of fish and bivalve molluscs of the Caspian Sea. They are accompanied by the necessary information on the taxonomy, morphology and distribution of families, genera and species, as well as an overview of their fishing or resource significance and role in the ecosystem. In order to understand the specifics of the Caspian fauna of fish and molluscs, the book opens with a physical and geographical survey of the Caspian Sea with an emphasis on fluctuations of its level, and a sketch on the sea geological history. Special attention is paid to identifying the historical paths and the main periods of formation of the fauna, including the modern period, during which there was enrichment of the fauna of the sea by invasive species of different taxonomic groups. The book includes a glossary of terms needed to understand the terminology associated with morphology, general zoology, taxonomy and the study of invasions.