This report addresses the conservation of freshwater ecosystems within the United States. A comparison of the pre-settlement and current conditions of U.S. freshwater resources is provided. Requirements for maintaining freshwater ecosystem physical and biotic components, such as flow patterns, sediment/organic inputs, temperature, light, chemical/nutrient conditions, and plant/animal assemblages are discussed. Freshwater ecosystem management case studies on the Colorado River, the Great Lakes, and South Florida are presented. The report also elaborates on the need to balance human use against needs of freshwater ecosystems. Issues in Ecology is an ongoing series of reports designed to present major ecological issues in an easy-to-read manner. This Issue summarizes the consensus of a panel of scientific experts based on the information that was current and available at the time of its publication in 2003.