The United States has made significant progress in cleaning its rivers, lakes, and oceans. Investment in wastewater treatment plant technology, conservation practices with land managers, and restoration of natural systems is working in many places. The public supports clean water, yet there is still a long way to go in achieving the vision of fishable, swimmable waters. More than half of the country's streams, lakes, and estuaries are not meeting the water quality standards established under the Clean Water Act to provide clean drinking water, recreation, fish and wildlife habitat, and other designated uses. The work that lies ahead to achieve clean water will require additional tools and new approaches that can account for watershed dynamics, allow flexibility on how to achieve clear, enforceable goals, and target investment where it can most effectively improve water quality. Water quality trading, under the right conditions, can fit these criteria.