This report summarizes water quality assessments submitted by the states to EPA under section 305(b) of the Clean Water Act. The report finds that the states assessed 16 percent of the nation's 3.5 million river and stream miles, 39 percent of its 41.7 million acres of lakes, ponds and reservoirs, and 29 percent of its 87,791 estuary square miles. Forty-four percent of assessed river and stream miles, 64 percent of assessed lake acres, and 30 percent of assessed estuary square miles were found to be impaired for one or more of the uses designated for them by the states. Leading causes of impairment included pathogens, mercury, nutrients, and organic enrichment/low dissolved oxygen. Top sources of impairment included atmospheric deposition, agriculture, hydrologic modifications, and unknown or unspecified sources. This report is a companion to electronically-submitted state water quality information available on EPA's Web site, known as ATTAINS, at http://www.epa.gov/waters/ir. In addition to viewing the national summary and information by state at this Web site, users can click down to the individual waterbody level to find out more about water quality conditions.