The Japanese Archipelago (land area: 377 880 km2) extends over a distance of c. 2000 km, with a maximum width of 300 km. Geologically, it is a young and tectonically very active area. Japan is a mountainous, wet and forested country, with its people concentrated in densely populated urban areas along the coast and on alluvial plains. Rivers are short (max. length: 370 km), steep, and exhibit flashy flow regimes. The river regime coefficient ranges from 200 to 400, which is up to an order-of-magnitude higher than that of most continental rivers. Japan has a rich freshwater fauna and flora with a high proportion of endemic species. A distinct latitudinal gradient (subarctic to subtropical climate) in combination with the radial character of the river network results in high spatial differentiation of the freshwater fauna. While water quality has improved remarkably during the past decades, Japanese rivers are still heavily impacted by canalization, loss of most dynamic flood plains, flow regulation, invasion by exotic species, and intensive urbanization. Currently 49% of the entire human population concentrates on 14% of the land, and the annual flood damage is the highest worldwide. As a consequence, major recent restoration initiatives aim to protect people and property against floods as well as simultaneously improving the ecological integrity of river ecosystems.