City is one of the important anthropogenic modifications of land; therefore its hydrological aspects must be studied from the view point of environmental change. Extension of impermeable land surface modifies the runoff processes and water circulation of the area. City needs much clear water for the citizen and many kinds of activities, although the polluted water must be eliminated possibly soon. Therefore, urban hydrology should be studied from two stand points, namely, modification of runoff by built-up area and transport of municipal waters. In this article, these factors are explained based on examples in Tokyo area. It is also emphasized that cultural and historical aspects are also necessary to understand the water environment of a city.