The concept of minimal in-stream flows dates from at least the mid-1970s. It is an important issue to everyone who withdraws ground or surface waters, injects water underground, or adds water to streams and rivers, particularly in the drought-stricken western US. At the federal level, the US EPA funded a grant to define ecological and related flows and create methods to measure them and the US Geological Survey developed measurement methods. Several years ago Oregon established statutory requirements that peak and ecological flows be maintained in any projects funded by state grants or other assistance. New Zealand, Denmark, India, and other countries either have incorporated such stream and river flow requirements in national laws or are in the process of doing so. Consideration of these flows is now a requirement for funding by the World Bank. This makes water allocation decisions even more difficult.