This book reviews and summarizes the studies on the fish diversity of Japan. It covers the present knowledge of ichthyofauna, habitat distribution, phylogeography, ecology, morphology, and conservation, as well as the history of ichthyology and fish collections in Japan. The book comprises five parts: I. Fish Diversity and Ichthyology of Japan, II. Habitat Distribution and Species Diversity, III. Diversity within Species: Phylogeographic Perspective on Japanese Fishes, IV. Morphological and Ecological Diversifications, and V. Conservation of Fish Diversity in Japan. The Japanese Archipelago is surrounded by two major warm and one cold currents. It is located in the western North Pacific and encompasses several climatic regimes from north to south. Although the land area of Japan is small, the Exclusive Economic Zone (EEZ) of Japan ranks as the sixth largest in the world, including several marginal seas (Sea of Okhotsk, Sea of Japan, and East China Sea), and deep trenches (Izu-Ogasawara, Japan, and Kurile Trenches). Owing to a variety of marine habitats and a complex geological history, Japan has a rich fish species diversity, representing over 4,500 species in 370 families. The richness of fish species diversity has attracted many scientists since the late 1700s, and continuous studies have led to the development of ichthyology in Japan. With chapters written by leading experts in the field, the book will provide a stimulating and reliable resource for future research and contribute to the progress of ichthyology of the world.