In 2001, the National Water-QualityAssessment (NAWQA) Program of the U.S. Geological Survey (USGS) began its second decade of studies. A total of 42 study units (major river basins and aquifers across the nation) will be reassessed in three groups of 14 on a rotating schedule. Each group of study units will be studied intensively for three years, followed by six years of low-intensity assessment. One of the primary goals in the second decade is to improve understanding of the key processes that control water-quality conditions in order to establish the links among the sources of contaminants, their transport through the hydrologic system, and the effects of contaminants and physical alterations on stream biota and ecosystems and on the quality of drinking water. An improved understanding of these links will provide the basis for predicting water-quality conditions in unmonitored areas and for predicting the likely effects of contemplated changes in land- and water-management practices.