The first two decades of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program have provided a successful and useful assessment of U.S. water-quality conditions, how they have changed over time, and how natural features and human activities have affected those conditions. Now, planning is underway for the third decade (Cycle 3) of the Program outlined in the Science Plan, with challenges including ensuring that the NAWQA remain a national program in the face of declining resources, balancing new activities against long-term studies, and maintaining focus amidst numerous and competing stakeholder demands. The Science Plan for Cycle 3 articulates a forward-thinking vision for NAWQA science over the next decade, building on the previous cycles' data, experience, and products. Preparing for the Third Decade (Cycle 3) of the National Water-Quality Assessment (NAWQA) Program explains the national needs outlined in the plan, NAWQA's need to emphasize collaboration with other USGS and external programs, other federal agencies, state and local governments, and the private sector.