In the last decades, insights from the fields of ecology, geomorphology, and hydrology have been applied to the question of the streamflows necessary for environmental maintenance. For instance, determining the streamflow needed for spawning by salmon or trout requires ascertaining how much water, for how long, and at what time it will be needed? And what flows are necessary for the sustenance of streamside vegetation? Answers to these and similar questions have been sought to minimize environmental degradation in the development or relicensing of water projects, in restoring riverine ecosystems, and in balancing multiple uses for limited water resources. In this contribution, the varieties of environmental maintenance flows applied to rivers are described, as are their fundamental principles. These environmental maintenance flows include flows to maintain aesthetics and recreation, streambed sediment size and its mobility, the channel, its features and continuity, and the floodplain, its wetness regime, and riparian vegetation.